Organic electroluminescent element, display device, lighting device, π-conjugated compound, and light-emitting thin film

ABSTRACT

An object of the present invention is to provide an organic electroluminescent element containing an organic layer interposed between an anode and a cathode, the organic layer containing at least one light emitting layer, wherein the at least one light emitting layer contains a π-conjugated compound having an electron donor portion and an electron acceptor portion in the molecule; the π-conjugated compound has a direction vector from an atom having a HOMO orbital in the electron donor portion to an electron cloud of the HOMO orbital, and a direction vector from an atom having a LUMO orbital in the electron acceptor portion to an electron cloud of the LUMO orbital, and the two direction vectors form an angle θ in the range of 90 to 180 degrees; and the π-conjugated compound has a plurality of the electron donor portions or a plurality of the electron acceptor portions.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.15/329,851, filed on Jan. 27, 2017, which was a 371 of PCT/JP2015/071638filed on Jul. 30, 2015, which, in turn, claimed the priority of JapanesePatent Application No. JP 2014-155852 filed on Jul. 31, 2014, eachapplication is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an organic electroluminescent element.Further, the present invention relates to a display device and alighting device provided with the organic electroluminescent element, aπ-conjugated compound and a light-emitting thin film containing theπ-conjugated compound. More specifically, the present invention relatesto an organic electroluminescent element achieving improved lightemitting efficiency.

BACKGROUND

Organic electroluminescent (hereinafter referred to as “EL”) elements(also referred to as “organic electroluminescence elements”), which arebased on electroluminescence of organic materials, have already been putinto practice as a new generation of light emitting systems capable ofachieving planar light emission. Organic EL elements have recently beenapplied to electronic displays and also to lighting devices and displaydevices. Thus, it has been demanded further development of organic ELelements.

As an emission mode of an organic EL, there are two types. One is “aphosphorescence emission type” which emits light when a triplet excitedstate returns to a ground state, and another one is “a fluorescenceemission type” which emits light when a singlet excited state returns toa ground state.

When an electric filed is applied to an organic EL element, a hole andan electron are respectively injected from an anode and a cathode, theyare recombined in a light emitting layer to produce an exciton. At thismoment, a singlet exciton and a triplet exciton are formed with a ratioof 25%:75%. Therefore, it is known that a phosphorescence emission typeusing a triplet exciton will produce theoretically high internal quantumefficiency compared with a fluorescence emission type.

However, in order to obtain high quantum efficiency in a phosphorescenceemission type, it is required to use a complex compound having a raremetal of iridium or platinum in the center metal. This may induce anindustrial problem of the amount of deposits or the cost of the raremetals in the future.

On the other hand, in recent years, new techniques relevant to afluorescence emission type have been proposed to improve emissionefficiency.

For example, Patent document 1 discloses a technique which is focused ona phenomenon wherein singlet excitons are generated by collision of twotriplet excitons (it is called as Triplet-Triplet Annihilation (TTA), orTriplet-Triplet Fusion (TTF)), and which improves the emissionefficiency of a fluorescent element by allowing the TTA phenomenon tooccur effectively. Although this technique can increase power efficiencyof a fluorescence emission material (hereafter, it is called as afluorescent emission material or fluorescent material) from two to threetimes larger than the power efficiency of a conventional fluorescentmaterial, the emission efficiency in TTA is not as high as that of theaforementioned phosphorescent material due to a theoretical limitation,because the rate of conversion of the excited triplet energy level tothe excited singlet energy level will remain to about 40%.

Recent studies have disclosed a fluorescent material that employs athermally activated delayed fluorescent mechanism (hereinafter alsoreferred to as “TADF”). It is reported that it may be applied to anorganic EL element (for example, refer to Patent document 2 andNon-patent documents 1 to 2). By making use of this delayed fluorescencecaused by the TADF mechanism, theoretically, it is possible to achievean internal quantum efficiency of 100% in fluorescence emission, whichis similar to the phosphorescent emission.

In order to make appear the TADF phenomenon, it is required that areverse intersystem crossing from the triplet state, which is producedwith an amount of 75% by an electric field excitation in an amount of75% at room temperature or at an emission layer temperature on theemission device, to the singlet state should be taken place. Further, bythe mechanism that the singlet exciton produced by the reverseintersystem crossing emits fluorescence in the same way as the singletexciton produced with an amount of 25%, it is theoretically possible torealize 100% internal quantum efficiency. In order to make appear thisreverse intersystem crossing, it is necessary that the absolute value ofthe difference between the singlet excited level and the triplet excitedlevel (hereafter, it is called as ΔE_(ST)) is very small. To obtain aminimum ΔE_(ST) in an organic compound, it is preferable that a HOMO anda LUMO in the molecule are not mixed and localized respectively.

For example, in the case of 2CzPN illustrated in “a” of FIG. 1, a HOMOis localized at a carbazolyl group at the 1 position and the 2 positionof the benzene ring, and a LUMO is localized at cyano groups at the 4position and the 5 position. As a result, the HOMO and the LUMO of 2CzPNmay be separated, and ΔE_(ST) becomes very small as indicated in “b” ofFIG. 1. Thus a TADF phenomenon will be produced. On the other hand, inthe case of 2CzXy (“a” of FIG. 2) which is produced by substitutingcyano groups at the 4 position and the 5 position of 2CzPN with methylgroups, the HOMO and the LUMO cannot be clearly separated as is seen in2CzPN. As a result, ΔE_(ST) cannot be made small, and a TADF phenomenonwill not be produced.

Further, it is known that an addition of the third component (anassist-dopant compound) which exhibits a TADF property into a lightemitting layer composed of a host compound and an emission compound iseffective to achieve high efficiency (Non-patent document 3). Byproducing 25% of singlet exciton and 75% of triplet exciton via anelectric field excitation on an assist-dopant compound, the tripletexciton will produce the singlet exciton through the reverse intersystemcrossing (RISC). The energy of the singlet exciton will be moved to theemission compound via an energy transfer. It is possible that theemission compound emits light. Consequently, theoretically, it ispossible to emit light from the emission compound by making use of 100%of the exciton. It may achieve high emission efficiency.

However, the localization of the HOMO and the LUMO, which is arequirement for making appear the TADF phenomenon, will form an excitedstate having an intermolecular charge transfer (CT) property. This willbecome a factor of broadening an absorption spectrum or an emissionspectrum. This broadening phenomenon becomes a fatal problem for a colordesigning of an organic EL element. The reason of this problem will bedescribed in the following,

An electronic state of 2CzPN is schematically illustrated in “a” of FIG.3. A molecule known as a TADF emission material (hereafter, it may becalled as “a TADF compound”) has a localized HOMO and a localized LUMO,and it has an imbalanced charge in the molecule. This charge imbalancewill induce imbalance in the medium substance (for example, a solvent ora host compound, see “b” of FIG. 3). Therefore, as indicated in “c” ofFIG. 3, the medium substance will be electrostatically adsorbed to theTADF compound. Interactions will be formed at a variety of positions anddirections. As a result, an energy distribution in the excited state ofthe TADF compound will be spread, and it is known that an absorptionspectrum or an emission spectrum will be broadened.

On the other hand, FIG. 4 illustrates a schematic diagram of aninteraction between a phosphorescent compound and a host compound. Asillustrated in “a” of FIG. 4, the phosphorescent compound (Ir(ppy)₃) hasa localized HOMO and a localized LUMO respectively placed in the innerportion and in the outer portion of the molecule. Since the HOMO portionexists substantially at an iridium metal in the center of the complex,it will not contribute to an electrostatic interaction with thesurrounding medium. The LUMO distributed in the ligand will interactwith the host compound (“b” of FIG. 4). Since it is localized in theouter side of the molecule, the location and direction of theinteraction will be limited. Consequently, an energy distribution in theexcited state of the phosphorescent compound will be restrained, andbroadening of an absorption spectrum and an emission spectrum willbecome small compared with a conventional TADF compound (“c” of FIG. 4).

Accordingly, it is required a new molecular design for a TADF compoundenabling to restrain an energy distribution in the excited state as thephosphorescent compound. A conventional organic EL element has notachieved all of the following properties at the same time: restrain ofbroadening of an absorption spectrum and an emission spectrum, highemission efficiency, and non-use of a rare metal.

PRIOR ART DOCUMENTS Patent Documents

Patent document 1: WO 2010/134350

Patent document 2: JP-A No. 2013-116975

Non-Patent Documents

-   Non-patent document 1: H. Uoyama, et al., Nature, 2012, 492,    234-238.-   Non-patent document 2: Q. Zhang et al., Nature, Photonics, 2014, 8,    326-332.-   Non-patent document 3: H. Nakanotani, et al., Nature Communication,    2014, 5, 4016-4022.

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The present invention has been made in view of the above-describedproblems and situation. An object of the present invention is to providean organic electroluminescent element enabling to achieve restrainedbroadening of an absorption spectrum and an emission spectrum, and highemission efficiency without using a rare metal. An object of the presentinvention is to provide a display device and a lighting device providedwith the organic electroluminescent element. Further, an object of thepresent invention is to provide a π-conjugated compound enabling toachieve restrained broadening of an absorption spectrum and an emissionspectrum, and high emission efficiency without using a rare metal, and alight-emitting thin film containing the π-conjugated compound.

Means to Solve the Problems

The present inventors have investigated the cause of the above-describedproblems in order to solve the problems. It was found to provide anorganic EL element capable of improving emission efficiency byincorporating a specific π-conjugated compound having a donor portionand an acceptor portion each being placed in a specific positionalrelationship in the molecule in at least one of the light-emittinglayer.

That is, the above-described problems of the present invention aresolved by the following embodiments.

1. An organic electroluminescent element comprising an organic layerinterposed between an anode and a cathode, the organic layer containingat least one light emitting layer,

wherein the at least one light emitting layer contains a π-conjugatedcompound having an electron donor portion and an electron acceptorportion in the molecule;

the π-conjugated compound has a direction vector from an atom having aHOMO orbital in the electron donor portion to an electron cloud of theHOMO orbital, and a direction vector from an atom having a LUMO orbitalin the electron acceptor portion to an electron cloud of the LUMOorbital, and the two direction vectors form an angle θ in the range of90 to 180 degrees; and the π-conjugated compound has at least one of aplurality of the electron donor portions and a plurality of the electronacceptor portions.

2. The organic electroluminescent element described in the embodiment 1,

wherein the angle θ is in the range of 135 to 180 degrees.

3. The organic electroluminescent element described in the embodiments 1or 2,

wherein one of the electron acceptor portions is bonded to two or moreelectron donor portions through a linking group, or one of the electrondonor portions is bonded to two or more electron acceptor portionsthrough a linking group.

4. The organic electroluminescent element described in the embodiments 1or 2,

wherein one of the electron acceptor portions is directly bonded to twoor more electron donor portions, or one of the electron donor portionsis directly bonded to two or more electron acceptor portions.

5. The organic electroluminescent element described in any one of theembodiments 1 to 4,

wherein the at least one light emitting layer contains a π-conjugatedcompound represented by any one of Formulas (1) to (8).

In Formulas, X¹ to X⁸ and Y¹ to Y²⁰ each respectively represent theelectron donor portion or the electron acceptor portion; when X¹ to X⁸each respectively represent the electron donor portion, Y¹ to Y²⁰ eachrespectively represent the electron acceptor portion; when X¹ to X⁸ eachrespectively represent the electron acceptor portion, Y¹ to Y²⁰ eachrespectively represent the electron donor portion; L¹ to L¹⁰ represent alinking group, L¹ to L¹⁰ each respectively represent an aryl group whichmay have a substituent or a heteroaryl group which may have asubstituent, L¹ binds X¹ and Y¹ through adjacent carbon atoms, L² bindsX¹ and Y² through adjacent carbon atoms, L³ binds X¹ and Y³ throughadjacent carbon atoms, L⁴ binds X¹ and Y⁴ through adjacent carbon atoms,L⁵ binds X² and Y⁵ through adjacent carbon atoms, L⁶ binds X² and Y⁶through adjacent carbon atoms, L⁷ binds X² and Y⁷ through adjacentcarbon atoms, L⁸ binds X³ and Y⁸ through adjacent carbon atoms, L⁹ bindsX³ and Y⁹ through adjacent carbon atoms, and L¹⁰ binds X⁷ and Y¹⁹through adjacent carbon atoms.

6. The organic electroluminescent element described in the embodiment 5,wherein the electron donor portion and the electron acceptor portionrepresented by X¹ to X⁸ and Y¹ to Y²⁰ in Formulas (1) to (8) eachrespectively are one selected from the group consisting of an aryl groupwhich may have a substituent, a heteroaryl group which may have asubstituent, an alkyl group which may have a substituent, a carbonylgroup which may have a substituent, a nitrogen atom which may have asubstituent, a sulfur atom which may have a substituent, a boron atomwhich may have a substituent, a phosphor atom which may have asubstituent, an oxygen atom which may have a substituent, and a siliconatom which may have a substituent.7. The organic electroluminescent element described in the embodiments 5or 6,

wherein L¹ to L¹⁰ in Formulas (1) to (3) and (7) each are a benzenering.

8. The organic electroluminescent element described in any one of theembodiments 1 to 7,

wherein an absolute value of a difference between a lowest excitedsinglet energy level and a lowest excited triplet energy level (ΔE_(ST))is 0.5 eV or less.

9. The organic electroluminescent element described in any one of theembodiments 1 to 8,

wherein the at least one light emitting layer contains:

the π-conjugated compound; and at least one of a fluorescent compoundand a phosphorescent compound.

10. The organic electroluminescent element described in any one of theembodiments 1 to 9,

wherein the at least one light emitting layer contains:

the π-conjugated compound; at least one of a fluorescent compound and aphosphorescent compound; and a host compound.

11. A display device provided with the organic electroluminescentelement described in any one of the embodiments 1 to 10.

12. A lighting device provided with the organic electroluminescentelement described in any one of the embodiments 1 to 10.

13. A π-conjugated compound having an electron donor portion and anelectron acceptor portion in the molecule,

wherein the π-conjugated compound has a direction vector from an atomhaving a HOMO orbital in the electron donor portion to an electron cloudof the HOMO orbital, and a direction vector from an atom having a LUMOorbital in the electron acceptor portion to an electron cloud of theLUMO orbital, and the two direction vectors form an angle θ in the rangeof 90 to 180 degrees; and the π-conjugated compound has a plurality ofthe electron donor portions or a plurality of the electron acceptorportions.

14. A light-emitting thin film containing the π-conjugated compounddescribed in the embodiment 13.

Effects of the Invention

By the above-described embodiments of the present invention, it ispossible to provide an organic electroluminescent element enabling toachieve restrained broadening of an absorption spectrum and an emissionspectrum, and high emission efficiency without using a rare metal. It isalso possible to provide a display device and a lighting device providedwith the organic electroluminescent element. Further, it is possible toprovide a π-conjugated compound enabling to achieve restrainedbroadening of an absorption spectrum and an emission spectrum, and highemission efficiency without using a rare metal, and a light-emittingthin film containing the π-conjugated compound.

A formation mechanism or an action mechanism of the effects of thepresent invention is not clearly identified, but it is supposed asfollows.

The present invention is specifically effective when the above-describedangle θ is in the range of 90 to 180 degrees.

In FIG. 5, “a-1” to “a-3” and “b” each are a schematic drawingillustrating a π-conjugated compound containing one electron donorportion and one electron acceptor portion for convenience. It will bedescribed the case in which the angle θ formed with a direction vectorof a donor portion and a direction vector of an acceptor portion is inthe range of the present invention, and the case in which the angle θ isoutside the range of the present invention by making use of these “a-1”to “a-3” and “b” in FIG. 5. Here, arrows in “a-1” to “a-3” and “b” ofFIG. 5 each represent: a direction vector from an atom having a HOMOorbital in the electron donor portion to an electron cloud of the HOMOorbital; or a direction vector from an atom having a LUMO orbital in theelectron acceptor portion to an electron cloud of the LUMO orbital.

In “a-1” to “a-3” of FIG. 5 indicating the angle θ in the range of thepresent invention, an electron transfer in space will easily occur froma HOMO of the donor portion to a LUMO of the acceptor portion. As aresult, high emission efficiency will be achieved in an organic ELelement. On the other hand, when the angle θ is outside the range of thepresent invention as illustrated in “b” of FIG. 5, an electron transferin space will hardly occur from the HOMO of the donor portion to theLUMO of the acceptor portion. As a result, high emission efficiency willnot be achieved.

In the following, it will be described the restraining effect ofbroadening of an absorption spectrum or an emission spectrum. In “a” ofFIG. 6, there is illustrated for convenience a simplified schematicdrawing of a π-conjugated compound containing one electron donor portionand two electron acceptor portions, and having an angle θ within therange of the present invention. A HOMO distributed in a donor portion ofa TADF compound is facing to a LUMO in an acceptor portion. There is nospace where the HOMO will interact with the surrounding medium.Consequently, as illustrated in “b” of FIG. 6, the position and thedirection of the interaction between the π-conjugated compound (the TADFmolecule) of the present invention and the medium (the host compound)will be limited compared with the case of “c” in FIG. 3. As a result, itis produced a restraining effect of broadening of an absorption spectrumor an emission spectrum.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an energy diagram illustrating ΔE_(ST).

FIG. 2 is another example of an energy diagram illustrating ΔE_(ST).

FIG. 3 is a schematic interaction diagram of a TADF compound and a hostcompound.

FIG. 4 is a schematic diagram illustrating an interaction of aphosphorescent compound and a host compound.

FIG. 5 is a schematic diagram illustrating an angle θ of the presentinvention.

FIG. 6 is a schematic diagram illustrating an interaction of aπ-conjugated compound of the present invention and a medium.

FIG. 7 is a schematic diagram illustrating the case in which aπ-conjugated compound has a function of an assist-dopant.

FIG. 8 is a schematic diagram illustrating the case in which aπ-conjugated compound has a function of a host compound.

FIG. 9 is a schematic diagram illustrating an example of a displaydevice including an organic EL element.

FIG. 10 is a schematic diagram of a display device by an active matrixmode.

FIG. 11 is a schematic view illustrating a pixel circuit.

FIG. 12 is a schematic diagram of a display device by a passive matrixmode.

FIG. 13 is a schematic view of a lighting device.

FIG. 14 is a cross-sectional diagram of a lighting device.

FIG. 15 is a schematic diagram illustrating a calculation method of anangle θ of the present invention.

EMBODIMENTS TO CARRY OUT THE INVENTION

An organic electroluminescent element of the present invention ischaracterized in having the following features. It comprises an organiclayer interposed between an anode and a cathode, the organic layercontaining at least one light emitting layer, wherein the at least onelight emitting layer contains a π-conjugated compound having an electrondonor portion and an electron acceptor portion in the molecule; theπ-conjugated compound has a direction vector from an atom having a HOMOorbital in the electron donor portion to an electron cloud of the HOMOorbital, and a direction vector from an atom having a LUMO orbital inthe electron acceptor portion to an electron cloud of the LUMO orbital,and the two direction vectors form an angle θ in the range of 90 to 180degrees; and the π-conjugated compound has a plurality of the electrondonor portions or a plurality of the electron acceptor portions.

The above-described features are technical features commonly owned bythe invention according to the embodiments 1 to 14.

From the viewpoint of obtaining an effect of the present invention, apreferable embodiment of the present invention is that theabove-described angle θ is in the range of 135 to 180 degrees. By thisembodiment, an electron transfer in space will easily occur from thedonor portion to the acceptor portion. As a result, emission efficiencywill be further improved. This is a preferable embodiment.

A preferable embodiment of the present invention is that one of theelectron acceptor portions is bonded to two or more electron donorportions through a linking group, or one of the electron donor portionsis bonded to two or more electron acceptor portions through a linkinggroup. This embodiment will restrain broadening of an absorptionspectrum or an emission spectrum.

Another preferable embodiment of the present invention is that one ofthe electron acceptor portions is directly bonded to two or moreelectron donor portions, or one of the electron donor portions isdirectly bonded to two or more electron acceptor portions. Thisembodiment will restrain broadening of an absorption spectrum or anemission spectrum.

Another preferable embodiment of the present invention is that the atleast one light emitting layer contains a π-conjugated compoundrepresented by any one of Formulas (1) to (8). This embodiment ispreferable from the viewpoint of achieving high emission efficiency.

Another preferable embodiment of the present invention is that theelectron donor portion and the electron acceptor portion represented byX¹ to X⁸ and Y¹ to Y²⁰ in Formulas (1) to (8) each respectively are oneselected from the group consisting of an aryl group which may have asubstituent, a heteroaryl group which may have a substituent, an alkylgroup which may have a substituent, a carbonyl group which may have asubstituent, a nitrogen atom which may have a substituent, a sulfur atomwhich may have a substituent, a boron atom which may have a substituent,a phosphor atom which may have a substituent, an oxygen atom which mayhave a substituent, and a silicon atom which may have a substituent.This embodiment is preferable from the viewpoint of achieving highemission efficiency.

Another preferable embodiment of the present invention is that L¹ to L¹⁰in Formulas (1) to (3) and (7) each are a benzene ring. This embodimentis preferable from the viewpoint of achieving high emission efficiency.

Another preferable embodiment of the present invention is that anabsolute value of a difference between a lowest excited singlet energylevel and a lowest excited triplet energy level (ΔE_(ST)) is 0.5 eV orless. This embodiment is preferable from the viewpoint of easilyachieving an intersystem crossing.

Another preferable embodiment of the present invention is that the atleast one light emitting layer contains: the π-conjugated compound; andat least one of a fluorescent compound and a phosphorescent compound.This embodiment is preferable from the viewpoint of achieving highemission efficiency.

Another preferable embodiment of the present invention is that the atleast one light emitting layer contains: the π-conjugated compound; atleast one of a fluorescent compound and a phosphorescent compound; and ahost compound. This embodiment is preferable from the viewpoint ofachieving high emission efficiency.

An organic electroluminescent element of the present invention issuitably incorporated in a display device. This is a preferableembodiment because a display device having high emission efficiency willbe obtained.

An organic electroluminescent element of the present invention issuitably incorporated in a lighting device. This is a preferableembodiment because a lighting device having high emission efficiencywill be obtained.

A π-conjugated compound of the present invention is characterized inhaving an electron donor portion and an electron acceptor portion in themolecule, wherein the π-conjugated compound has a direction vector froman atom having a HOMO orbital in the electron donor portion to anelectron cloud of the HOMO orbital, and a direction vector from an atomhaving a LUMO orbital in the electron acceptor portion to an electroncloud of the LUMO orbital, and the two direction vectors form an angle θin the range of 90 to 180 degrees; and the π-conjugated compound has atleast one of a plurality of the electron donor portions and a pluralityof the electron acceptor portions. It may be provided a material havinghigh emission efficiency by this embodiment.

A π-conjugated compound of the present invention may be suitablyincorporated in a light-emitting thin film. It will be obtained alight-emitting thin film having high efficiency by this embodiment.

The present invention and the constitution elements thereof, as well asconfigurations and embodiments, will be detailed in the following. Inthe present description, when two figures are used to indicate a rangeof value before and after “to”, these figures are included in the rangeas a lowest limit value and an upper limit value.

<Light Emission Mode of Organic EL>

As a light emission mode of an organic EL, there are two types. One is“a phosphorescence emission type” which emits light when a tripletexcited state returns to a ground state, and another one is “afluorescence emission type” which emits light when a singlet excitedstate returns to a ground state.

When excitation is done by an electric field such as in the case of anorganic EL element, a triplet exciton is produced with a probability of75%, and a singlet exciton is produced with a probability of 25%.Consequently, it is possible that a phosphorescent emission has higheremission efficiency than fluorescent emission. The phosphorescentemission is an excellent mode to realize low electric consumption.

On the other hand, with respect to the fluorescent emission, it wasfound a method of using a TTA mechanism in which singlet excitons aregenerated from two triplet excitons (it is called as Triplet-TripletAnnihilation (TTA), or Triplet-Triplet Fusion (TTF)) to improve theemission efficiency. The TTA mechanism may be achieved by the tripletexcitons produced with a probability of 75%, which will normally takethe route of radiationless deactivation only to produce heat. By makingthe triplet excitons to be produced in a high density, the TTA mechanismis effective.

In recent years, the group of Adachi found the following phenomenon. Byachieving a small energy gap between the singlet excited state and thetriplet excited state, it is allowed to occur a reverse intersystemcrossing from the triplet state of lower energy level to the singletstate. This may be done by the Joule heat produced during the emissionand/or the environmental temperature in which the light emission elementis placed. As a result, it may be achieved a fluorescent emission in ayield of nearly 100% (it is called as a thermally activated delayedfluorescence: TADF). And it was found a compound enabling to occur thisphenomenon (refer to Non-patent document 1, for example).

<Phosphorescence Emission Material>

As described above, although the phosphorescence emission hastheoretically an advantage of 3 times of the fluorescence emission, anenergy deactivation (=phosphorescence emission) from the triplet excitedstate to the singlet ground state is a forbidden transition. In the samemanner, the intersystem crossing from the singlet excited state to thetriplet excited state is also a forbidden transition. Consequently, itsrate constant is usually small. That is, since the transition takesplace hardly, the lifetime of the exciton becomes long such as an orderof millisecond or second. As a result, it is difficult to obtain arequired emission.

However, when an emission occurs from a complex including a heavy atomof iridium or platinum, the rate constant of the above-describedforbidden transition becomes larger by 3 orders due to the heavy metaleffect of the center metal. It is possible to obtain a phosphorescencequantum efficiency of 100% when selection of the ligand is properlydone.

However, in order to obtain an ideal emission, it is required to use arare metal such as iridium or palladium, or a noble metal such asplatinum. If a large amount of these metals are used, the reserves andthe price of these metal will become problem.

<Fluorescence Emission Material>

A common fluorescence emission material is not required to be a heavymetal complex as in the case of a phosphorescence emission material. Itmay be applied a so-called organic compound composed of a combination ofelements such as carbon, oxygen, nitrogen and hydrogen. Further, anon-metallic element such as phosphor, sulfur, and silicon may be used.And a complex of typical element such as aluminum or zinc may be used.The variation of the materials is almost without limitation.

However, the conventional fluorescence emission material will use only25% of the excitons to light emission. Therefore, it cannot be expectedhigh emission efficiency as achieved in phosphorescence emission.

<Delayed Fluorescent Material>

[Excited Triplet-Triplet Annihilation (TTA) Delayed FluorescentMaterial]

A light emission mode employing a delayed fluorescence appeared to solvethe problem of the fluorescent material. The TTA mode originated fromthe collision of the compounds at a triplet state may be described inthe following Scheme. That is, in the past, a part of the tripletexciton is only converted to heat. This energy of the exciton is changedto a singlet exciton via an intersystem crossing to result incontributing to the light emission. In a practical organic EL element,it was proved that external quantum efficiency was double of theconventional fluorescent element.T*+T*→S*+S  Scheme:(In the Scheme, T* represents a triplet exciton, S* represents a singletexciton, and S represents a ground state molecule.)

However, as can be seen from the above-described Scheme, only onesinglet exciton is generated from two triplet excitons. Consequently,theoretically, 100% internal quantum efficiency cannot be obtained basedon this mode.

[Thermally Activated Delayed Fluorescent (TADF) Compound]

A TADF mode, which is another type of high efficient fluorescenceemission, is a mode enabling to resolve the problem.

A fluorescent material has an advantage of being molecular-designedwithout imitation as described above. Among the molecular-designedcompounds, there are specific compounds having an energy leveldifference (hereafter, it is indicated as ΔE_(ST)) between a tripletexcited state and a singlet excited state being in very close vicinity(refer to “a” in FIG. 1).

In spite of that fact that these compounds don't contain a heavy metalatom in the molecule, there occurs a reverse intersystem crossingreaction from the triplet excited state to the singlet excited state dueto the small ΔE_(ST) value. This reaction will not usually occur.Further, since the rate constant of the deactivation from the singletexcited state to the ground state (=fluorescence emission) is extremelyhigh, the triplet state will likely return to the ground state via thesinglet state while emitting fluorescence, instead of thermallydeactivating (radiationless deactivation) to the ground state. As aresult, in TADF mechanism, ideally, it is possible to realizefluorescence emission of 100%.

<Molecular designing idea concerning ΔE_(ST)>

A molecular designing idea to reduce the ΔE_(ST) will be described.

In order to reduce the value of ΔE_(ST), theoretically the mosteffective way is to minimize the spatial overlaps of the highestoccupied molecular orbital (HOMO) and the lowest unoccupied molecularorbital (LUMO).

Generally, in the electronic orbitals of the molecule, it is known thatHOMO has a distribution to an electron donating position and LUMO has adistribution to an electron withdrawing position. By introducing anelectron donating structure and an electron withdrawing structure in themolecule, it is possible to keep apart the positions in which HOMO andLUMO exist.

For example, Applied Physics vol. 82, no. 6, 2013 “OrganicPhoto-electronics in the commercialization stage” discloses thefollowing. By introducing an electron withdrawing structure such as acyano group, a sulfonyl group or a triazine group, and an electrondonating structure such as a carbazole group or a diphenyl amino group,LUMO and HOMO are respectively made localized.

In addition, it is also effective to minimize the molecular structurechange between the ground state and the triplet excited state of themolecule. As a means to minimize the structure change, it can cite acompound having an inflexible structure. Here, inflexibility indicatesthe state in which freely movable portions in the molecule are notabundant caused by preventing a free rotation of the bond between therings in the molecule, or by introducing a condensed ring having a largeπ-conjugate plane, for example. In particular, by making the portionparticipating in the light emission to be rigid, it is possible tominimize the molecular structure change in the excited state.

<Common Problem Possessed by TADF Compound>

A TADF compound possesses a variety of problems arisen from the aspectsof the light emission mechanism and the molecular structure.

A part of common problems possessed by a TADF compound will be describedin the following.

In a TADF compound, it is required to keep apart the portions in whichHOMO and LUMO exist as much as possible in order to minimize ΔE_(ST).For this reason, the electronic state of the molecule becomes almostnear the intra molecular CT state (intramolecular charge transferstate).

When a plurality of these molecules exist, these molecules will bestabilized by making in proximity the donor portion in one molecule andthe acceptor portion in other molecule. This stabilized condition isformed not only with 2 molecules, but it may be formed with 3 and 5molecules. Consequently, there are produced a variety of stabilizedconditions having a broad distribution. The shape of absorption spectrumor the emission spectrum will be broad. Further, even if a multiplemolecular aggregation of 2 or more molecules does not formed, there maybe formed a variety of existing conditions having different interactiondirections or angles of two molecules. As a result, basically, the shapeof absorption spectrum or the emission spectrum will be broad.

When the emission spectrum becomes broad, it will generate two majorproblems. One is a problem of decreasing the color purity of theemission color. This is not so important when it is applied to anillumination use. However, when it is used for an electronic device, thecolor reproduction region becomes small. And the color reproduction ofpure colors will become decreased. As a result, it is difficult to applyto a commercial product.

Another problem is the shortened wavelength of the rising wavelength inthe short wavelength side of the emission spectrum (it is called as“fluorescent zero-zero band”). That is, the S₁ level becomes high(becoming higher energy level of the excited singlet energy).

When the fluorescent zero-zero band becomes shortened, thephosphorescent zero-zero band derived from T₁ (being lower than S₁) willbecome shortened (becoming higher T₁).

Therefore, the host compound is required to have high S₁ and high T₁ inorder to prevent the reverse energy transfer from the dopant.

This is a major problem. A host compound basically made of an organiccompound will take plural and unstable chemical species conditions suchas a cationic radical state, an anionic radical state and an excitedstate in an organic EL element. These chemical species may be madeexisted in relatively stable condition by expanding a π-conjugate systemin the molecule.

Further, in a TADF compound without containing a heavy metal, thetransition from the triplet excited state to the ground state isforbidden transition. The existing time at the triplet excited state(exciton lifetime) is extremely long such as in an order of severalhundred microsecond to millisecond. Therefore, even if the T₁ energylevel of the host compound is higher than that of the light emittingmaterial, it will be increased the probability of taking place a reverseenergy transfer from the triplet excited state of the light emittingmaterial to the host compound due to the long lifetime. As a result, itis difficult to sufficiently make occur a required reverse intersystemcrossing from the triplet excited state to the singlet excited state ofthe TADF compound. Instead, there occurs an unrequired reverse energytransfer to the host compound as a major route to result in failure toobtain insufficient emission efficiency.

In order to solve the above-described problem, it is required to makesharp a shape of an emission spectrum of the TADF compound, and todecrease the difference between the emission maximum wavelength and therise of the emission spectrum. This may be achieved basically byreducing the change of the molecular structure of the singlet excitedstate and the triplet excited state.

Further, in order to prevent the reverse energy transfer to the hostcompound, it is effective to shorten the existing time of the tripletexcited state of the TADF compound (exciton lifetime). In order torealize this, the possible ways to solve the problem are: to minimizethe molecular structure change between the ground state and the tripletexcited state; and to introduce a suitable substituent or an element toloosen the forbidden transition.

It will be described a variety of measuring methods concerning aπ-conjugated compound according to the present invention.

[Electron Density Distribution]

From the viewpoint of decreasing ΔE_(ST), it is preferable that aπ-conjugated compound according to the present invention has a HOMO anda LUMO substantially separated with each other in the molecule. Thedistribution state of the HOMO and the LUMO may be obtained from theelectron density distribution in the optimized structure by a molecularorbital calculation.

The structure optimization and the calculation of the electron densitydistribution of the π-conjugated compound of the present invention witha molecular orbital calculation may be done by employing a software of amolecular orbital calculation using B3LYP as a functional and 6-31G(d)as a base function for a calculation method. There is no limitation tothe software, the same results may be obtained with any software.

In the present invention, as a molecular orbital calculation software,it was used Gaussian 09 made by The US Gaussian Inc., (Revision C.01, byM. J. Frisch et al., Gaussian Inc., 2010).

Here, the condition of “a HOMO and a LUMO being substantially separated”indicates the state in which the center portion of the HOMO orbitaldistribution and the center portion of the LUMO orbital distributioncalculated with the above-described molecular calculation method areseparated. More preferably, the HOMO orbital distribution and the LUMOorbital distribution are substantially not superimposed.

The separation state of the electron density distribution of the HOMOand the LUMO may be determined by making calculation of excited stateswith a Time-dependent DFT method starting from the optimized structurecalculation using B3LYP as a functional and 6-31G(d) as a base functionas described above. The excited state energy levels of S₁ and T₁ areobtained, and ΔE_(ST) is calculated from the scheme of:ΔE_(ST)=E(S₁)−E(T₁). The smaller the calculated ΔE_(ST), it indicatesthat the HOMO and the LUMO are more separated. In the present invention,an absolute value of ΔE_(ST) obtained by the above-described calculationmethod is preferably 0.5 eV or less, more preferably it is 0.2 eV orless, and still more preferably it is 0.1 eV or less.

[Lowest Excited Singlet Energy Level S₁]

In the present invention, the lowest excited singlet energy S1 of theπ-conjugated compound of the present invention may be determined by acommon technique. Specifically, a target compound is deposited onto aquartz substrate to prepare a sample, and an absorption spectrum of thesample is measured at ambient temperature (300 K) (vertical axis:absorbance, horizontal axis: wavelength). A tangential line is drawn atthe rising point of the absorption spectrum on the longer wavelengthside, and the lowest excited singlet energy is calculated by a specificconversion expression on the basis of the wavelength at the point ofintersection of the tangential line with the horizontal axis.

When the π-conjugated compound used in the present invention has a highaggregation property as a molecule itself, it is likely to causemolecular aggregation, and thus a thin film prepared from the compoundmay cause a measurement error due to molecular aggregation. In thepresent invention, the lowest excited singlet energy level is determinedfrom, as an approximation, the peak wavelength of emission of a solutionof the π-conjugated compound at room temperature (about 25° C.) inconsideration of a relatively small Stokes shift of the π-conjugatedcompound and a very small structural change of the compound between theexcited state and the ground state. This determination process may use asolvent which does not affect the molecular aggregation state of theπ-conjugated compound; for example, a non-polar solvent having a smallsolvent effect, such as cyclohexane or toluene.

[Lowest Excited Triplet Energy Level T₁]

The lowest excited triplet energy level (T₁) of the π-conjugatedcompound of the present invention is determined on the basis of thephotoluminescent (PL) properties of a solution or thin film of thecompound. For example, a thin film is prepared from a dilute dispersionof the π-conjugated compound, and the transient PL properties of thethin film are determined with a streak camera for separation of afluorescent component and a phosphorescent component to determine theabsolute value of the energy difference ΔE_(ST) therebetween. The lowestexcited triplet energy level may be obtained from the lowest excitedsinglet energy level.

For measurement and evaluation, the absolute PL quantum yield wasdetermined with an absolute PL Quantum yield measuring apparatusC9920-02 (manufactured by Hamamatsu Photonics K.K.). The emissionlifetime was determined with a streak camera C4334 (manufactured byHamamatsu Photonics K.K.) under excitation of the sample with a laserbeam.

<<Constitution Layers of Organic EL Element>>

Representative element constitutions used for an organic EL element ofthe present invention are as follows, however, the present invention isnot limited to these.

(1) Anode/light emitting layer/cathode

(2) Anode/light emitting layer/electron transport layer/cathode

(3) Anode/hole transport layer/light emitting layer/cathode

(4) Anode/hole transport layer/light emitting layer/electron transportlayer/cathode

(5) Anode/hole transport layer/light emitting layer/electron transportlayer/electron injection layer/cathode

(6) Anode/hole injection layer/hole transport layer/light emittinglayer/electron transport layer/cathode

(7) Anode/hole injection layer/hole transport layer/(electron blockinglayer/) light emitting layer/(hole blocking layer/) electron transportlayer/electron injection layer/cathode

Among these, the embodiment (7) is preferably used. However, the presentinvention is not limited to this.

The light emitting layer of the present invention is composed of one ora plurality of layers. When a plurality of layers are employed, it maybe placed a non-light emitting intermediate layer between the lightemitting layers.

According to necessity, it may be provided with a hole blocking layer(it is also called as a hole barrier layer) or an electron injectionlayer (it is also called as a cathode buffer layer) between the lightemitting layer and the cathode. Further, it may be provided with anelectron blocking layer (it is also called as an electron barrier layer)or an hole injection layer (it is also called as an anode buffer layer)between the light emitting layer and the anode.

An electron transport layer according to the present invention is alayer having a function of transporting an electron. An electrontransport layer includes an electron injection layer, and a holeblocking layer in a broad sense. Further, an electron transport layerunit may be composed of plural layers.

A hole transport layer according to the present invention is a layerhaving a function of transporting a hole. A hole transport layerincludes a hole injection layer, and an electron blocking layer in abroad sense. Further, a hole transport layer unit may be composed ofplural layers.

In the representative element constitutions as described above, thelayers eliminating an anode and a cathode are also called as “organiclayers”.

(Tandem Structure)

An organic EL element of the present invention may be so-called a tandemstructure element in which plural light emitting units each containingat least one light emitting are laminated.

A representative example of an element constitution having a tandemstructure is as follows.

Anode/first light emitting unit/intermediate layer/second light emittingunit/intermediate layer/third light emitting unit/cathode.

Here, the above-described first light emitting unit, second lightemitting unit, and third light emitting unit may be the same ordifferent. It may be possible that two light emitting units are the sameand the remaining one light emitting unit is different.

The plural light emitting units each may be laminated directly or theymay be laminated through an intermediate layer. Examples of anintermediate layer are: an intermediate electrode, an intermediateconductive layer, a charge generating layer, an electron extractionlayer, a connecting layer, and an intermediate insulating layer. Knowncomposing materials may be used as long as it can form a layer which hasa function of supplying an electron to an adjacent layer to the anode,and a hole to an adjacent layer to the cathode.

Examples of a material used in an intermediate layer are: conductiveinorganic compounds such as ITO (indium tin oxide), IZO (indium zincoxide), ZnO₂, TiN, ZrN, HfN, TiO_(x), VO_(x), CuI, InN, GaN, CuAlO₂,CuGaO₂, SrCu₂O₂, LaB₆, RuO₂, and Al; a two-layer film such as Au/Bi₂O₃;a multi-layer film such as SnO₂/Ag/SnO₂, ZnO/Ag/ZnO, Bi₂O₃/Au/Bi₂O₃,TiO₂/TiN/TiO₂, and TiO₂/ZrN/TiO₂; fullerene such as C₆₀; and aconductive organic layer such as oligothiophene, metal phthalocyanine,metal-free phthalocyanine, metal porphyrin, and metal-free porphyrin.The present invention is not limited to them.

Examples of a preferable constitution in the light emitting unit are theconstitutions of the above-described (1) to (7) from which an anode anda cathode are removed. However, the present invention is not limited tothem.

Examples of a tandem type organic EL element are described in: U.S. Pat.Nos. 6,337,492, 7,420,203, 7,473,923, 6,872,472, 6,107,734, 6,337,492,WO 2005/009087, JP-A 2006-228712, JP-A 2006-24791, JP-A 2006-49393, JP-A2006-49394, JP-A 2006-49396, JP-A 2011-96679, JP-A 2005-340187, JPPatent 4711424, JP Patent 3496681, JP Patent 3884564, JP Patent 4213169,JP-A 2010-192719, JP-A 2009-076929, JP-A 2008-078414, JP-A 2007-059848,JP-A 2003-272860, JP-A 2003-045676, and WO 2005/094130. Theconstitutions of the elements and the composing materials are describedin these documents, however, the present invention is not limited tothem.

Each layer that constitutes an organic EL element of the presentinvention will be described in the following.

<<Light Emitting Layer>>

A light emitting layer according to the present invention is a layerwhich provide a place of emitting light via an exciton produce byrecombination of electrons and holes injected from an electrode or anadjacent layer. The light emitting portion may be either within thelight emitting layer or at an interface between the light emitting layerand an adjacent layer thereof. The constitution of the light emittinglayer according to the present invention is not particularly limited aslong as it satisfies the requirements of the present invention.

A total thickness of the light emitting layer is not particularlylimited. However, in view of layer homogeneity, required voltage duringlight emission, and stability of the emitted light color against a driveelectric current, the total layer thickness is preferably adjusted to bein the range of 2 nm to 5 μm, more preferably, it is in the range of 2to 500 nm, and still most preferably, it is in the range of 5 to 200 nm.

Each light emitting layer is preferably adjusted to be in the range of 2nm to 1 μm, more preferably, it is in the range of 2 to 200 nm, andstill most preferably, it is in the range of 3 to 150 nm.

It is preferable that the light emitting layer of the present inventionincorporates a light emitting dopant (a light emitting dopant compound,a dopant compound, or simply called as a dopant) and a host compound (amatrix material, a light emitting host compound, or simply called as ahost). When at least one of the light emitting layers contains aπ-conjugated compound and at least one of fluorescent compound and aphosphorescent compound, the emission efficiency is improved. It ispreferable. Further, when at least one of the light emitting layerscontains: a π-conjugated compound; at least one of fluorescent compoundand a phosphorescent compound; and a host compound, the emissionefficiency is improved. It is also preferable.

(1) Light Emitting Dopant

As a light emitting dopant, it is preferable to employ: a fluorescenceemitting dopant (also referred to as a fluorescent dopant and afluorescent compound) and a phosphorescence emitting dopant (alsoreferred to as a phosphorescent dopant and a phosphorescent emittingmaterial). In the present invention, it is preferable that at least onelight emitting layer contains a fluorescence emitting dopant. In thepresent invention, it is preferable that at least one of the lightemitting layers contains a fluorescent compound (described later) and aπ-conjugated compound served as an assist-dopant.

In the present invention, it is preferable that the light emitting layercontains a light emitting compound in the range of 0.1 to 50 mass %,more preferably in the range of 1 to 30 mass %.

A concentration of a light emitting compound in a light emitting layermay be arbitrarily decided based on the specific compound employed andthe required conditions of the device. A concentration of a lightemitting compound may be uniform in a thickness direction of the lightemitting layer, or it may have any concentration distribution.

It may be used plural light emitting compounds of the present invention.It may be used a combination of fluorescent compounds each having adifferent structure, or a combination of a fluorescence emittingcompound and a phosphorescence emitting compound. Any required emissioncolor will be obtained by this.

When the light emitting layer contains: a π-conjugated compound of thepresent invention having an absolute value of a difference between alowest singlet excited level and a lowest triplet level (ΔE_(ST)) is 0.5eV or less; a light emitting compound; and a host compound, theπ-conjugated compound of the present invention acts as an assist-dopant.Whereas, when the light emitting layer contains a π-conjugated compoundof the present invention and a light emitting compound withoutcontaining a host compound, the π-conjugated compound of the presentinvention acts as a host compound.

The mechanism of appearing the effects is the same for both cases. Thespecific feature is that a triplet exciton produced on the π-conjugatedcompound of the present invention is converted to a singlet exciton viaa reverse intersystem crossing (RISC).

By this, all energy of the excitons produced on the π-conjugatedcompound of the present invention is theoretically transferred to thelight emitting compound. It may be achieved high emission efficiency.

Consequently, when the light emitting layer contains 3 components of aπ-conjugated compound of the present invention, a light emittingcompound, and a host compound, it is preferable that the energy levelsof S₁ and T₁ of the π-conjugated compound are lower than the energylevels of S₁ and T₁ of the host compound, and higher than the energylevels of S₁ and T₁ of the light emitting compound.

In the same manner, when the light emitting layer contains 2 componentsof a π-conjugated compound of the present invention and a light emittingcompound, it is preferable that the energy levels of S₁ and T₁ of theπ-conjugated compound are higher than the energy levels of S₁ and T₁ ofthe light emitting compound.

FIG. 7 and FIG. 8 illustrate a schematic diagram of the case in whichthe π-conjugated compound of the present invention acts as anassist-dopant or a host compound. FIG. 7 and FIG. 8 are only an example,the production process of the triplet exciton on the π-conjugatedcompound of the present invention is not limited to the electric fieldexcitation, the production process includes the cases of an energytransfer or an electron transfer in the light emitting layer or from thesurrounding interface.

Further, FIG. 7 and FIG. 8 illustrate the diagram using a fluorescenceemitting compound as a light emitting compound, however, the presentinvention is not limited to it, and it may be used a phosphorescenceemitting compound, and it may be used both of a fluorescence emittingcompound and a phosphorescence emitting compound.

When a π-conjugated compound of the present invention is used as anassist-dopant, it is preferable that the light emitting layer contains ahost compound in an amount of 100 mass % or more with respect to theπ-conjugated compound, and that it contains a fluorescence emittingcompound and/or a phosphorescence emitting compound in an amount of 0.1to 50 mass % with respect to the π-conjugated compound.

When a π-conjugated compound of the present invention is used as a hostcompound, it is preferable that the light emitting layer contains afluorescence emitting compound and/or a phosphorescence emittingcompound in an amount of 0.1 to 50 mass % with respect to theπ-conjugated compound.

When a π-conjugated compound of the present invention is used as anassist-dopant or a host compound, it is preferable that an emissionspectrum of the π-conjugated compound of the present invention and anabsorption spectrum of the light emitting compound are overlapped fromthe viewpoint of achieving high light emission efficiency.

Color of light emitted by an organic EL element or a compound of thepresent invention is specified as follows. In FIG. 3.16 on page 108 of“Shinpen Shikisai Kagaku Handbook (New Edition Color Science Handbook)”(edited by The Color Science Association of Japan, Tokyo Daigaku ShuppanKai, 1985), values determined via Spectroradiometer CS-1000 (produced byKonica Minolta, Inc.) are applied to the CIE chromaticity coordinate,whereby the color is specified.

In the present invention, it is preferable that the organic EL elementof the present invention exhibits white emission by incorporating one orplural light emitting layers containing plural emission dopants havingdifferent emission colors.

The combination of emission dopants producing white is not specificallylimited. It may be cited, for example, combinations of: blue and orange;and blue, green and red.

It is preferable that “white” in the organic EL element of the presentinvention shows chromaticity in the CIE 1931 Color Specification Systemat 1,000 cd/m² in the region of x=0.39±0.09 and y=0.38±0.08, whenmeasurement is done to 2-degree viewing angle front luminance via theaforesaid method.

(1.1) π-Conjugated Compound

The π-conjugated compound of the present invention has a directionvector from an atom having a HOMO orbital in the electron donor portionto an electron cloud of the HOMO orbital, and a direction vector from anatom having a LUMO orbital in the electron acceptor portion to anelectron cloud of the LUMO orbital, and the two direction vectors forman angle θ in the range of 90 to 180 degrees, and the π-conjugatedcompound has at least one of a plurality of the electron donor portionsand a plurality of the electron acceptor portions. It is more preferablethat the angle θ is in the range of 135 to 180 degrees.

This π-conjugated compound may be suitably used for a light-emittingthin film of the present invention described later.

A donor portion is a porting having an electron donating property. Inthe present invention, a HOMO designates a π-orbital or an n-orbitallocalized in a donor portion. Here, “portion” in the donor portionindicates a substituent or an atomic group.

Examples of a donor portion are: arylamine derivatives, carbazole,phenoxazine, 9,10-dihydroacrydine, and phenothiazine.

An acceptor portion is an electron withdrawing portion that is electrondeficient. In the present invention, a LUMO designates a π*-orbital or aσ*-orbital localized in an acceptor portion. Here, “portion” in theacceptor portion indicates a substituent or an atomic group.

Examples of an acceptor portion are: a benzene ring substituted with acyano group, a triazine ring, a pyrimidine ring, a boron atom, and asulfonyl group.

It will be described a direction vector from an atom having a HOMOorbital in the electron donor portion to an electron cloud of the HOMOorbital, and a direction vector from an atom having a LUMO orbital inthe electron acceptor portion to an electron cloud of the LUMO orbital.The electron cloud of the HOMO orbital in the electron donor portionindicates an electron cloud of a π-orbital or an n-orbital in the donorportion. The electron cloud of the LUMO orbital in the electron acceptorportion indicates an electron cloud of a π*-orbital or a σ*-orbital inthe acceptor portion. The directions of π-orbital, n-orbital, π*-orbitaland σ*-orbital extended from the atom are known by the molecular orbitalmethod. For example, the π-orbital composed of sp2 hybrid orbitals has adirection of a 2 pz orbital as a direction vector of the presentinvention.

The angle θ according to the present invention is an angle formed with adirection vector from an atom having a HOMO orbital in the electrondonor portion to an electron cloud of the HOMO orbital, and a directionvector from an atom having a LUMO orbital in the electron acceptorportion to an electron cloud of the LUMO orbital.

The angle θ according to the present invention may be calculated using amolecular orbital calculation software of Gaussian 09 made by The USGaussian Inc., (Revision C.01, by M. J. Frisch et al., Gaussian Inc.,2010) with B3LYP as a functional and 6-31G(d) as a base function for acalculation method. The calculation software and the calculation methodare not limited, it may be obtained the same results by using anymethod.

The π-conjugated compound of the present invention has an electron donorportion and an electron acceptor portion in the molecule. It ispreferable that the π-conjugated compound has one electron acceptorportion that is bonded to two or more electron donor portions through alinking group, or it has one electron donor portion that is bonded totwo or more electron acceptor portions through a linking group.

Further, it is also preferable that the π-conjugated compound of thepresent invention has one electron acceptor portion that is directlybonded to two or more electron donor portions, or it has one electrondonor portion that is directly bonded to two or more electron acceptorportions.

Specific examples of a preferable π-conjugated compound of the presentinvention are a π-conjugated compound represented by any one of Formulas(1) to (8). At least one of the light emitting layers according to thepresent invention preferably contains at least one of these π-conjugatedcompounds.

In Formulas, X¹ to X⁸ and Y¹ to Y²⁰ each respectively represent theelectron donor portion or the electron acceptor portion; when X¹ to X⁸each respectively represent the electron donor portion, Y¹ to Y²⁰ eachrespectively represent the electron acceptor portion; when X¹ to X⁸ eachrespectively represent the electron acceptor portion, Y¹ to Y²⁰ eachrespectively represent the electron donor portion; L¹ to L¹⁰ represent alinking group, L¹ to L¹⁰ each respectively represent an aryl group whichmay have a substituent or a heteroaryl group which may have asubstituent, L¹ binds X¹ and Y¹ through adjacent carbon atoms, L² bindsX¹ and Y² through adjacent carbon atoms, L³ binds X¹ and Y³ throughadjacent carbon atoms, L⁴ binds X¹ and Y⁴ through adjacent carbon atoms,L⁵ binds X² and Y⁵ through adjacent carbon atoms, L⁶ binds X² and Y⁶through adjacent carbon atoms, L⁷ binds X² and Y⁷ through adjacentcarbon atoms, L⁸ binds X³ and Y⁸ through adjacent carbon atoms, L⁹ bindsX³ and Y⁹ through adjacent carbon atoms, and L¹ binds X⁷ and Y1⁹ throughadjacent carbon atoms.

Further, it is preferable that the electron donor portion and theelectron acceptor portion represented by X¹ to X⁸ and Y¹ to Y²⁰ inFormulas (1) to (8) each respectively are one selected from the groupconsisting of an aryl group which may have a substituent, a heteroarylgroup which may have a substituent, an alkyl group which may have asubstituent, a carbonyl group which may have a substituent, a nitrogenatom which may have a substituent, a sulfur atom which may have asubstituent, a boron atom which may have a substituent, a phosphor atomwhich may have a substituent, an oxygen atom which may have asubstituent, and a silicon atom which may have a substituent.

Further, it is preferable that the electron acceptor portionsrepresented by X¹ to X⁸ and Y¹ to Y²⁰ in Formulas (1) to (8) eachrespectively are one selected from the group consisting of: an aryl ringhaving 6 to 20 carbon atoms which may by partially substituted with analkyl group, an aryl ring having 6 to 20 carbon atoms which may bypartially substituted with an alkoxy group, a carbazole ring which mayhave a substituent, an indoloindole ring which may have a substituent, a9,10-dihydroacrydine ring which may have a substituent, a phenoxazinering which may have a substituent, a phenothiazine ring which may have asubstituent, a 5,10-dihydrophenazine ring which may have a substituent,a dibenzothiophene ring which may have a substituent, an amino groupwhich may have a substituent, and a thio group which may have asubstituent.

Further, it is preferable that the electron acceptor portionsrepresented by X¹ to X⁸ and Y¹ to Y²⁰ in Formulas (1) to (8) eachrespectively are one selected from the group consisting of: an aryl ringhaving 6 to 20 carbon atoms which may be partially substituted with acyano group, an aryl ring having 6 to 20 carbon atoms which may bepartially substituted with a fluoroalkyl group, an aryl ring having 6 to20 carbon atoms which may be partially or wholly substituted with afluorine atom, a nitrogen atom-containing aromatic ring having 5 to 13carbon atoms which may have a substituent, a dibenzoborol ring which mayhave a substituent, a dibenzothiphene oxide ring which may have asubstituent, a dibenzothiphene dioxide ring

which may have a substituent, a sulfinyl group which may have asubstituent, a sulfonyl group which may have a substituent, a borylgroup which may have a substituent, a phosphine oxide group which mayhave a substituent, a silyl group which may have a substituent, apyridine ring, a pyridazine ring, a pyrimidine ring, a pyrazine ring, atriazine ring, a quinoline ring, an isoquinoline ring, a quinazolinering, a cinnoline ring, a quinoxaline ring, a phthalazine ring, apteridine ring, an acridine ring, a phenanthridine ring, and aphenanthroline ring. It is particularly preferable that they are oneselected from the group consisting of: a pyridine ring, a pyridazinering, a pyrimidine ring, a pyrazine ring, a triazine ring, a quinolinering, an isoquinoline ring, a phenanthridine ring, and a phenanthrolinering.

Specific examples of the aryl group having 6 to 20 carbon atoms are: abenzene ring, an indene ring, a naphthalene ring, an azulene ring, afluorene ring, a phenanthrene ring, an anthracene ring, anacenaphthylene ring, a biphenylene ring, a chrysene ring, a naphthacenering, a pyrene ring, a pentalene ring, an aceanthrylene ring, aheptalene ring, a triphenylene ring, an as-indacene ring, a chrysenering, an s-indacene ring, a pleiadene ring, a phenalene ring, afluoranthene ring, a perylene ring, and an acephenanthrylene ring. Morepreferable examples are: a benzene ring, a naphthalene ring, a fluorenering, a phenanthrene ring, an anthracene ring, a biphenylene ring, achrysene ring, a pyrene ring, a triphenylene ring, a chrysene ring, afluoranthene ring, and a perylene ring. Particularly preferable examplesare: a benzene ring, a naphthalene ring, a phenanthrene ring, and apyrene ring.

The above-describe alkyl group may be straight, branched or cyclic.Examples thereof are: a straight, branched or cyclic alkyl group having1 to 20 carbon atoms. Specific examples are:

a methyl group, an ethyl group, an n-propyl group, an isopropyl group,an n-butyl group, an s-butyl group, a t-butyl group, an n-pentyl group,a neopentyl group, an n-hexyl group, a cyclohexyl group, a 2-ethylhexylgroup, an n-heptyl group, an n-octyl group, a 2-hexyloctyl group, ann-nonyl group, an n-decyl group, an n-undecyl group, an n-dodecyl group,an n-tridecyl group, an n-tetradecyl group, an n-pentadecyl group, ann-hexadecyl group, an n-heptadecyl group, an n-octadecyl group, ann-nonadecyl group, and an n-icosyl group. More preferable examples are:a methyl group, an ethyl group, an isopropyl group, a t-butyl group, acyclohexyl group, a 2-ethylhexyl group, and 2-hexyloctyl group.

The above-describe alkoxy group may be straight, branched or cyclic.Examples thereof are: a straight, branched or cyclic alkoxy group having1 to 20 carbon atoms. Specific examples are: a methoxy group, an ethoxygroup, an n-propoxy group, an isopropoxy group, an n-butoxy group, anisobutoxy group, a t-butoxy group, an n-pentyloxy group, a neopentyloxygroup, an n-hexyloxy group, a cyclohexyloxy group, an n-heptyloxy group,an n-octyloxy group, a 2-ethylhexyloxy group, a nonyloxy group, adecyloxy group, a 3,7-dimethyloctyloxy group, an n-undecyloxy group, ann-dodecyloxy group, an n-tridecyloxy group, an n-tetradecyloxy group, a2-n-hexyl-n-octyloxy group, an n-pentadecyloxy group, an n-hexadecyloxygroup, an n-heptadecyloxy group, an n-octadecyloxy group, ann-nonadecyloxy group, and an n-icosyloxy group. More preferable examplesare: a methoxy group, an ethoxy group, an isopropoxy group, a t-butoxygroup, a cyclohexyloxy group, a 2-ethylhexyloxy group, and a2-hexyloctyloxy group.

The above-describe linking group (including L¹ to L¹⁰ in Formulas (1) to(3) and (7)) is not limited in particular as long as it does not hinderthe effect of the present invention. Preferable examples thereof are: abenzene ring, a naphthalene ring, a thiophene ring, a furan ring, abenzofuran ring, a benzothiophene ring, and a thienothiophene ring. Itis particularly preferable that L¹ to L¹⁰ in Formulas (1) to (3) and (7)each are a benzene ring.

As a π-conjugated compound represented by any one of Formulas (1) to(8), it may be cited the following compounds. However, the presentinvention is not limited to them.

In addition, the following compounds all have the above-described angleθ in the range of 90 to 180 degrees.

By using these compounds, it is possible to achieve a structure in whichan electron transition will be easily taken place from a donor portionto an acceptor portion. In addition, among these compounds, thematerials having ΔE_(ST) (an absolute value) in the range of 0.5 eV orless may exhibit a TADF property. Further, since these compounds have abipolar property and they may be compatible with a variety of energylevels, they may be used as an emission host, and they may be suitablyused as a hole transport compound or an electron transport compound.Consequently, the use of these compounds is not limited to a lightemission layer, they may be used in the hole injection layer, a holetransport layer, an electron blocking layer, a hole blocking layer, anelectron transport layer, an electron injection layer, or anintermediate layer.

<Synthetic Method>

The above-described π-conjugated compound may be synthesized with themethods described in Non-patent document 2: Journal of OrganometallicChemistry, 2003, 680, 218-222 and WO 2011/8560 or by referring to themethods described in the references of these documents.

(1.2) Fluorescence Emitting Dopant

As a fluorescent dopant, it may be used a π-conjugated compound of thepreset invention. Otherwise, it may be suitably selected from the knownfluorescent dopants and delayed fluorescent dopants used in a lightemitting layer of an organic EL element.

As specific known fluorescence emitting dopants usable in the presentinvention, listed are compounds such as: an anthracene derivative, apyrene derivative, a chrysene derivative, a fluoranthene derivative, aperylene derivative, a fluorene derivative, an arylacetylene derivative,a styrylarylene derivative, a styrylamine derivative, an arylaminederivative, a boron complex, a coumarin derivative, a pyran derivative,a cyanine derivative, a croconium derivative, a squarium derivative, anoxobenzanthracene derivative, a fluorescein derivative, a rhodaminederivative, a pyrylium derivative, a perylene derivative, apolythiophene derivative, and a rare earth complex compound.

In addition, it has been developed a light emitting dopant utilizingdelayed fluorescence. It may be used a light emitting dopant utilizingthis type of fluorescence. Specific examples of utilizing delayedfluorescence are compounds described in: WO 2011/156793, JP-A2011-213643, and JP-A 2010-93181. However, the present invention is notlimited to them.

(1.3) Phosphorescence Emitting Dopant

A phosphorescence emitting dopant according to the present inventionwill be described.

The phosphorescence emitting dopant according to the present inventionis a compound which is observed emission from an excited triplet statethereof. Specifically, it is a compound which emits phosphorescence at aroom temperature (25° C.) and exhibits a phosphorescence quantum yieldof at least 0.01 at 25° C. The phosphorescence quantum yield ispreferably at least 0.1.

The phosphorescence quantum yield will be determined via a methoddescribed in page 398 of Bunko II of Dai 4 Han Jikken Kagaku Koza 7(Spectroscopy II of 4th Edition Lecture of Experimental Chemistry 7)(1992, published by Maruzen Co. Ltd.). The phosphorescence quantum yieldin a solution will be determined using appropriate solvents. However, itis only necessary for the phosphorescent dopant of the present inventionto exhibit the above phosphorescence quantum yield (0.01 or more) usingany of the appropriate solvents.

A phosphorescence dopant may be suitably selected and employed from theknown materials used for a light emitting layer for an organic ELelement.

Examples of a known phosphorescence dopant are compound described in thefollowing publications.

Nature 395, 151 (1998), Appl. Phys. Lett. 78, 1622 (2001), Adv. Mater.19, 739 (2007), Chem. Mater. 17, 3532 (2005), Adv. Mater. 17, 1059(2005), WO 2009/100991, WO 2008/101842, WO 2003/040257, US 2006/835469,US 2006/0202194, US 2007/0087321, US 2005/0244673, Inorg. Chem. 40, 1704(2001), Chem. Mater. 16, 2480 (2004), Adv. Mater. 16, 2003 (2004),Angew. Chem. Int. Ed. 2006, 45, 7800, Appl. Phys. Lett. 86, 153505(2005), Chem. Lett. 34, 592 (2005), Chem. Commun. 2906 (2005), Inorg.Chem. 42, 1248 (2003), WO 2009/050290, WO 2002/015645, WO 2009/000673,US 2002/0034656, U.S. Pat. No. 7,332,232, US 2009/0108737, US2009/0039776, U.S. Pat. Nos. 6,921,915, 6,687,266, US 2007/0190359, US2006/0008670, US 2009/0165846, US 2008/0015355, U.S. Pat. Nos.7,250,226, 7,396,598, US 2006/0263635, US 2003/0138657, US 2003/0152802,U.S. Pat. No. 7,090,928, Angew. Chem. Int. Ed. 47, 1 (2008), Chem.Mater. 18, 5119 (2006), Inorg. Chem. 46, 4308 (2007), Organometallics23, 3745 (2004), Appl. Phys. Lett. 74, 1361 (1999), WO 2002/002714, WO2006/009024, WO 2006/056418, WO 2005/019373, WO 2005/123873, WO2005/123873, WO 2007/004380, WO 2006/082742, US 2006/0251923, US2005/0260441, U.S. Pat. Nos. 7,393,599, 7,534,505, 7,445,855, US2007/0190359, US 2008/0297033, U.S. Pat. No. 7,338,722, US 2002/0134984,and U.S. Pat. No. 7,279,704, US 2006/098120, US 2006/103874, WO2005/076380, WO 2010/032663, WO 2008/140115, WO 2007/052431, WO2011/134013, WO 2011/157339, WO 2010/086089, WO 2009/113646, WO2012/020327, WO 2011/051404, WO 2011/004639, WO 2011/073149, JP-A2012-069737, JP Application No. 2011-181303, JP-A 2009-114086, JP-A2003-81988, JP-A 2002-302671 and JP-A 2002-363552.

Among them, preferable phosphorescence emitting dopants are organicmetal complexes containing Ir as a center metal. More preferable arecomplexes containing at least one coordination mode selected from ametal-carbon bond, a metal-nitrogen bond, a metal-oxygen bond and ametal-sulfur bond.

(2) Host Compound

A host compound according to the present invention is a compound whichmainly plays a role of injecting or transporting a charge in a lightemitting layer. In an organic EL element, an emission from the hostcompound itself is substantially not observed.

Among the compounds incorporated in the light emitting layer, a massratio of the host compound in the aforesaid layer is preferably at least20%.

Host compounds may be used singly or may be used in combination of twoor more compounds. By using plural host compounds, it is possible toadjust transfer of charge, thereby it is possible to achieve highefficiency of an organic EL element.

In the following, preferable host compounds used in the presentinvention will be described.

A host compound may be a π-conjugated compound used in the presentinvention as described above. However it is not specifically limited tothat. From the viewpoint of a reverse energy transfer, it is preferablethat the host compound has a larger excited energy level than an excitedsinglet energy level of the dopant compound. It is more preferable thatthe host compound has a larger excited triplet energy level than anexcited triplet energy level of the dopant.

A host compound bears the function of transfer of the carrier andgeneration of an exciton in the light emitting layer. Therefore, it ispreferable that the host compound will exist in all of the activespecies of a cation radical state, an anion radial state and an excitedstate, and that it will not make chemical reactions such asdecomposition and addition. Further, it is preferable that the hostmolecule will not move in the layer with an Angstrom level when anelectric current is applied.

In particular, when the jointly used light emitting dopant exhibits TADFemission, since the lifetime of the triplet excited state of the TADFmaterial is long, it is required an appropriate design of a molecularstructure to prevent the host compound from having a lower T₁ level suchas: the host compound has a high T₁ energy; the host compounds will notform a low T₁ state when aggregated each other; the TADF material andthe host compound will not form an exciplex; and the host compound willnot form an electromer by applying an electric field.

In order to satisfy the above-described requirements, it is requiredthat: the host compound itself has a high hopping mobility; the hostcompound has high hole hopping mobility; and the host compound has smallstructural change when it becomes a triplet excited state. As arepresentative host compound satisfying these requirements, preferablecompounds are: a compound having a high T₁ energy such as a carbazolestructure, an azacarbazole structure, a dibenzofuran structure, adibenzothiophene structure and an azadibenzofuran structure. Inparticular, when the light emitting layer contains a carbazolederivative, it will promote suitable carrier hopping in the lightemitting layer and suitable dispersion of the emitting material. Therebyit may be obtained the effect of improved emitting property and improvedstability of the thin layer. It is a preferable embodiment.

A host compound has a hole transporting ability or an electrontransporting ability, as well as preventing elongation of an emissionwavelength. In addition, from the viewpoint of stably driving an organicEL element at high temperature, it is preferable that a host compoundhas a high glass transition temperature (T) of 90° C. or more, morepreferably, has a Tg of 120° C. or more.

Here, a glass transition temperature (Tg) is a value obtained using DSC(Differential Scanning Colorimetry) based on the method in conformity toJIS-K-7121-2012.

A host compound suitably used in the present invention is a π-conjugatedcompound according to the present invention as described above. Thereason of this is that the π-conjugated compound of the presentinvention has a condensed ring structure and the π-electron cloud isextended. As a result, the compound has high carrier transport abilityand a high glass transition temperature (Tg). Further, the π-conjugatedcompound of the present invention has a high triplet energy (T₁), and itis appropriately used for an emission of short wavelength (namely,having large T₁ and S₁).

As specific examples of a known host compound used in an organic ELelement of the present invention, the compounds described in thefollowing Documents are cited. However, the present invention is not tothem.

Japanese patent application publication (JP-A) Nos. 2001-257076,2002-308855, 2001-313179, 2002-319491, 2001-357977, 2002-334786,2002-8860, 2002-334787, 2002-15871, 2002-334788, 2002-43056,2002-334789, 2002-75645, 2002-338579, 2002-105445, 2002-343568,2002-141173, 2002-352957, 2002-203683, 2002-363227, 2002-231453,2003-3165, 2002-234888, 2003-27048, 2002-255934, 2002-260861,2002-280183, 2002-299060, 2002-302516, 2002-305083, 2002-305084 and2002-308837; US Patent Application Publication (US) Nos. 2003/0175553,2006/0280965, 2005/0112407, 2009/0017330, 2009/0030202, 2005/0238919; WO2001/039234, WO 2009/021126, WO 2008/056746, WO 2004/093 207, WO2005/089025, WO 2007/063796, WO 2007/063754, WO 2004/107822, WO2005/030900, WO 2006/114966, WO 2009/086028, WO 2009/003898, WO2012/023947, JP-A 2008-074939, JP-A 2007-254297, EP 2034538, WO2011/055933, and WO 2012/035853. The specific host compounds which maybe used in the present invention are: Compounds H-1 to H-231 inparagraphs [0255] to [0293] of JP-A No. 2015-38941, or H-232 to H-236 asdescribed in the following. However, the host compounds in the presentinvention are not limited to them.

A preferable host compound used for the present invention may be a lowmolecular weight compound which has a molecular weight enabling to bepurified with sublimation, or it may be a polymer having a repeatingunit.

The low molecular weight compound has an advantage of obtaining a highlypurified material since it is possible to purify with sublimation. Themolecular weight thereof is not specifically limited as long as it ispossible to purify with sublimation. A preferable molecular weight is3,000 or less, and a more preferable molecular weight is 2,000 or less.

A polymer or an oligomer having a repeating unit has an advantage ofeasily forming a film with a wet process. In addition, since a polymerhas generally a high Tg, the polymer is preferable from the viewpoint ofheat resistivity.

<<Electron Transport Layer>>

An electron transport layer of the present invention is composed of amaterial having a function of transferring an electron. It is onlyrequired to have a function of transporting an injected electron from acathode to a light emitting layer.

A total layer thickness of the electron transport layer is notspecifically limited, however, it is generally in the range of 2 nm to 5μm, and preferably, it is in the range of 2 to 500 nm, and morepreferably, it is in the range of 5 to 200 nm.

In an organic EL element of the present invention, it is known thatthere occurs interference between the light directly taken from thelight emitting layer and the light reflected at the electrode located atthe opposite side of the electrode from which the light is taken out atthe moment of taking out the light which is produced in the lightemitting layer. When the light is reflected at the cathode, it ispossible to use effectively this interference effect by suitablyadjusting the total thickness of the electron transport layer in therange of several nm to several μm.

On the other hand, the voltage will be increased when the layerthickness of the electron transport layer is made thick. Therefore,especially when the layer thickness is large, it is preferable that theelectron mobility in the electron transport layer is 1×10⁻⁵ cm²/Vs ormore.

As a material used for an electron transport layer (hereafter, it iscalled as an electron transport material), it is only required to haveeither a property of ejection or transport of electrons, or a barrier toholes. Any of the conventionally known compounds may be selected andthey may be employed.

Cited examples thereof include: a nitrogen-containing aromaticheterocyclic derivative (a carbazole derivative, an azacarbazolederivative (a compound in which one or more carbon atoms constitutingthe carbazole ring are substitute with nitrogen atoms), a pyridinederivative, a pyrimidine derivative, a pyrazine derivative, a pyridazinederivative, a triazine derivative, a quinoline derivative, a quinoxalinederivative, a phenanthroline derivative, an azatriphenylene derivative,an oxazole derivative, a thiazole derivative, an oxadiazole derivative,a thiadiazole derivative, a triazole derivative, a benzimidazolederivative, a benzoxazole derivative, and a benzothiazole derivative); adibenzofuran derivative, a dibenzothiophene derivative, a silolederivative; and an aromatic hydrocarbon ring derivative (a naphthalenederivative, an anthracene derivative and a triphenylene derivative).

Further, metal complexes having a ligand of a 8-quinolinol structure ordibnenzoquinolinol structure such as tris(8-quinolinol)aluminum (Alq₃),tris(5,7-dichloro-8-quinolinol)aluminum,tris(5,7-dibromo-8-quinolinol)aluminum,tris(2-methyl-8-quinolinol)aluminum, tris(5-methyl-8-quinolinol)aluminumand bis(8-quinolinol)zinc (Znq); and metal complexes in which a centralmetal of the aforesaid metal complexes is substituted by In, Mg, Cu, Ca,Sn, Ga or Pb, may be also utilized as an electron transport material.

Further, a metal-free or metal phthalocyanine, or a compound whoseterminal is substituted by an alkyl group or a sulfonic acid group, maybe preferably utilized as an electron transport material. A distyrylpyrazine derivative, which is exemplified as a material for a lightemitting layer, may be used as an electron transport material. Further,in the same manner as used for a hole injection layer and a holetransport layer, an inorganic semiconductor such as an n-type Si and ann-type SiC may be also utilized as an electron transport material.

It may be used a polymer material introduced these compounds in thepolymer side-chain or a polymer material having any one of thesesubstance in a polymer main chain.

In an electron transport layer according to the present invention, it ispossible to employ an electron transport layer of a higher n property(electron rich) which is doped with impurities as a guest material. Asexamples of a dope material, listed are those described in each of JP-ANos. 4-297076, 10-270172, 2000-196140, 2001-102175, as well as in J.Appl. Phys., 95, 5773 (2004).

Although the present invention is not limited thereto, preferableexamples of a known electron transport material used in an organic ELelement of the present invention are compounds described in thefollowing publications.

U.S. Pat. Nos. 6,528,187, 7,230,107, US 2005/0025993, US 2004/0036077,US 2009/0115316, US 2009/0101870, US 2009/0179554, WO 2003/060956, WO2008/132085, Appl. Phys. Lett. 75, 4 (1999), Appl. Phys. Lett. 79, 449(2001), Appl. Phys. Lett. 81, 162 (2002), Appl. Phys. Lett. 81, 162(2002), Appl. Phys. Lett. 79, 156 (2001), U.S. Pat. No. 7,964,293, US2009/030202, WO 2004/080975, WO 2004/063159, WO 2005/085387, WO2006/067931, WO 2007/086552, WO 2008/114690, WO 2009/069442, WO2009/066779, WO 2009/054253, WO 2011/086935, WO 2010/150593, WO2010/047707, EP 2311826, JP-A 2010-251675, JP-A 2009-209133, JP-A2009-124114, JP-A 2008-277810, JP-A 2006-156445, JP-A 2005-340122, JP-A2003-45662, JP-A 2003-31367, JP-A 2003-282270, and WO 2012/115034.

As a preferable electron transport material, it may be cited an aromaticheterocyclic ring compound containing at least one nitrogen atom.Examples thereof are: a pyridine derivative, a pyrimidine derivative, apyrazine derivative, a triazine derivative, a dibenzofuran derivative, adibenzothiophene derivative, a carbazole derivative, an azacarbazolederivative, and a benzimidazole derivative. An electron transportmaterial may be used singly, or may be used in combination of pluralkinds of compounds.

<<Hole Blocking Layer>>

A hole blocking layer is a layer provided with a function of an electrontransport layer in a broad meaning. Preferably, it contains a materialhaving a function of transporting an electron, and having very smallability of transporting a hole. It will improve the recombinationprobability of an electron and a hole by blocking a hole whiletransporting an electron.

Further, a composition of an electron transport layer described abovemay be appropriately utilized as a hole blocking layer of the presentinvention when needed.

A hole blocking layer placed in an organic EL element of the presentinvention is preferably arranged at a location in the light emittinglayer adjacent to the cathode side.

A thickness of a hole blocking layer according to the present inventionis preferably in the range of 3 to 100 nm, and more preferably, in therange of 5 to 30 nm.

With respect to a material used for a hole blocking layer, the materialused in the aforesaid electron transport layer is suitably used, andfurther, the material used as the aforesaid host compound is alsosuitably used for a hole blocking layer.

<<Electron Injection Layer>>

An electron injection layer (it is also called as “a cathode bufferlayer”) according to the present invention is a layer which is arrangedbetween a cathode and a light emitting layer to decrease an operatingvoltage and to improve an emission luminance. An example of an electroninjection layer is detailed in volume 2, chapter 2 “Electrode materials”(pp. 123-166) of “Organic EL Elements and Industrialization Frontthereof (Nov. 30, 1998, published by N.T.S. Co. Ltd.)”.

In the present invention, an electron injection layer is providedaccording to necessity, and as described above, it is placed between acathode and a light emitting layer, or between a cathode and an electrontransport layer.

An electron injection layer is preferably a very thin layer. The layerthickness thereof is preferably in the range of 0.1 to 5 nm depending onthe materials used.

An election injection layer is detailed in JP-A Nos. 6-325871, 9-17574,and 10-74586. Examples of a material preferably used in an electioninjection layer include: a metal such as strontium and aluminum; analkaline metal compound such as lithium fluoride, sodium fluoride, orpotassium fluoride; an alkaline earth metal compound such as magnesiumfluoride; a metal oxide such as aluminum oxide; and a metal complex suchas lithium 8-hydroxyquinolate (Liq). It is possible to use the aforesaidelectron transport materials.

The above-described materials may be used singly or plural kinds may beused together in an election injection layer.

<<Hole Transport Layer>>

In the present invention, a hole transport layer contains a materialhaving a function of transporting a hole. A hole transport layer is onlyrequired to have a function of transporting a hole injected from ananode to a light emitting layer.

The total layer thickness of a hole transport layer of the presentinvention is not specifically limited, however, it is generally in therange of 0.5 nm to 5 μm, preferably in the range of 2 to 500 nm, andmore preferably in the range of 5 to 200 nm.

A material used in a hole transport layer (hereafter, it is called as ahole transport material) is only required to have any one of propertiesof injecting and transporting a hole, and a barrier property to anelectron. A hole transport material may be suitably selected from theconventionally known compounds.

Examples of a hole transport material include: a porphyrin derivative, aphthalocyanine derivative, an oxazole derivative, an oxadiazolederivative, a triazole derivative, an imidazole derivative, a pyrazolinederivative, a pyrazolone derivative, a phenylenediamine derivative, ahydrazone derivative, a stilbene derivative, a polyarylalkanederivative, a triarylamine derivative, a carbazole derivative, anindolocarbazole derivative, an isoindole derivative, an acene derivativeof anthracene or naphthalene, a fluorene derivative, a fluorenonederivative, polyvinyl carbazole, a polymer or an oligomer containing anaromatic amine in a side chain or a main chain, polysilane, and aconductive polymer or an oligomer (e.g., PEDOT:PSS, an aniline typecopolymer, polyaniline and polythiophene).

Examples of a triarylamine derivative include: a benzidine typerepresented by α-NPD (4,4′-bis[N-(1-naphthyl)-N-phenyamino]biphenyl), astar burst type represented by MTDATA(4,4′,4″-tris(N-(3-methylphenyl)-N-phenylamino)triphenylamine), acompound having fluorenone or anthracene in a triarylamine bonding core.

A hexaazatriphenylene derivative described in JP-A Nos. 2003-519432 and2006-135145 may be also used as a hole transport material.

In addition, it is possible to employ an electron transport layer of ahigher p property which is doped with impurities. As its example, listedare those described in each of JP-A Nos. 4-297076, 2000-196140, and2001-102175, as well as in J. Appl. Phys., 95, 5773 (2004).

Further, it is possible to employ so-called p-type hole transportmaterials, and inorganic compounds such as p-type Si and p-type SiC, asdescribed in JP-A No. 11-251067, and J. Huang et al. reference (AppliedPhysics Letters 80 (2002), p. 139). Moreover, an orthometal compoundshaving Ir or Pt as a center metal represented by Ir(ppy)₃ are alsopreferably used.

Although the above-described compounds may be used as a hole transportmaterial, preferably used are: a triarylamine derivative, a carbazolederivative, an indolocarbazole derivative, an azatriphenylenederivative, an organic metal complex, a polymer or an oligomerincorporated an aromatic amine in a main chain or in a side chain.

Specific examples of a known hole transport material used in an organicEL element of the present invention are compounds in the aforesaidpublications and in the following publications. However, the presentinvention is not limited to them.

Examples of a publication are: Appl. Phys. Lett. 69, 2160(1996), J.Lumin. 72-74, 985(1997), Appl. Phys. Lett. 78, 673(2001), Appl. Phys.Lett. 90, 183503(2007), Appl. Phys. Lett. 51, 913(1987), Synth. Met. 87,171(1997), Synth. Met. 91, 209(1997), Synth. Met. 111, 421(2000), SIDSymposium Digest, 37, 923(2006), J. Mater. Chem. 3, 319(1993), Adv.Mater. 6, 677(1994), Chem. Mater. 15, 3148(2003), US 2003/0162053, US2002/0158242, US 2006/0240279, US 2008/0220265, U.S. Pat. No. 5,061,569,WO 2007/002683, WO 2009/018009, EP 650955, US 2008/0124572, US2007/0278938, US 2008/0106190, US 2008/0018221, WO 2012/115034, JP-A2003-519432, JP-A 2006-135145, and U.S. patent application Ser. No.13/585,981.

A hole transport material may be used singly or may be used incombination of plural kinds of compounds.

<<Electron Blocking Layer>>

An electron blocking layer is a layer provided with a function of a holetransport layer in a broad meaning. Preferably, it contains a materialhaving a function of transporting a hole, and having very small abilityof transporting an electron. It will improve the recombinationprobability of an electron and a hole by blocking an electron whiletransporting a hole. Further, a composition of a hole transport layerdescribed above may be appropriately utilized as an electron blockinglayer of an organic EL element of the present invention when needed.

An electron blocking layer placed in an organic EL element of thepresent invention is preferably arranged at a location in the lightemitting layer adjacent to the anode side.

A thickness of an electron blocking layer is preferably in the range of3 to 100 nm, and more preferably, in the range of 5 to 30 nm.

With respect to a material used for an electron blocking layer, thematerial used in the aforesaid hole transport layer is suitably used,and further, the material used as the aforesaid host compound is alsosuitably used for an electron blocking layer.

<<Hole Injection Layer>>

A hole injection layer (it is also called as “an anode buffer layer”) isa layer which is arranged between an electrode and a light emittinglayer to decrease an operating voltage and to improve an emissionluminance. An example of a hole injection layer is detailed in volume 2,chapter 2 “Electrode materials” (pp. 123-166) of “Organic EL Elementsand Industrialization Front thereof (Nov. 30, 1998, published by N.T.S.Co. Ltd.)”.

A hole injection layer is provided according to necessity, and asdescribed above, it is placed between an anode and a light emittinglayer, or between an anode and a hole transport layer.

A hole injection layer is also detailed in JP-A Nos. 9-45479, 9-260062and 8-288069. Materials used in the hole injection layer are the samematerials used in the aforesaid hole transport layer.

Among them, preferable materials are: a phthalocyanine derivativerepresented by copper phthalocyanine; a hexaazatriphenylene derivativedescribed in JP-A Nos. 2003-519432 and 2006-135145; a metal oxiderepresented by vanadium oxide; a conductive polymer such as amorphouscarbon, polyaniline (or called as emeraldine) and polythiophene; anorthometalated complex represented by tris(2-phenylpyridine) iridiumcomplex; and a triarylamine derivative.

The above-described materials used in a hole injection layer may be usedsingly or plural kinds may be co-used.

<<Additive>>

The above-described organic layer of the present invention may furthercontain other additive.

Examples of an additive are: halogen elements such as bromine, iodineand chlorine, and a halide compound; and a compound, a complex and asalt of an alkali metal, an alkaline earth metal and a transition metalsuch as Pd, Ca and Na.

Although a content of an additive may be arbitrarily decided,preferably, it is 1,000 ppm or less based on the total mass of the layercontaining the additive, more preferably, it is 500 ppm or less, andstill more preferably, it is 50 ppm or less.

In order to improve a transporting property of an electron or a hole, orto facilitate energy transport of an exciton, the content of theadditive is not necessarily within these range, and other range ofcontent may be used.

<<Forming Method of Organic Layers>>

It will be described forming methods of organic layers according to thepresent invention (hole injection layer, hole transport layer, lightemitting layer, hole blocking layer, electron transport layer, andelectron injection layer).

Forming methods of organic layers according to the present invention arenot specifically limited. They may be formed by using a known methodsuch as a vacuum vapor deposition method and a wet method (wet process).

Examples of a wet process include: a spin coating method, a cast method,an inkjet method, a printing method, a die coating method, a bladecoating method, a roll coating method, a spray coating method, a curtaincoating method, and a LB method (Langmuir Blodgett method). From theviewpoint of getting a uniform thin layer with high productivity,preferable are method highly appropriate to a roll-to-roll method suchas a die coating method, a roll coating method, an inkjet method, and aspray coating method.

Examples of a liquid medium to dissolve or to disperse a material fororganic layers according to the present invention include: ketones suchas methyl ethyl ketone and cyclohexanone; aliphatic esters such as ethylacetate; halogenated hydrocarbons such as dichlorobenzene; aromatichydrocarbons such as toluene, xylene, mesitylene, and cyclohexylbenzene;aliphatic hydrocarbons such as cyclohexane, decalin, and dodecane;organic solvents such as DMF and DMSO.

These will be dispersed with a dispersion method such as an ultrasonicdispersion method, a high shearing dispersion method and a mediadispersion method.

A different film forming method may be applied to every organic layer.When a vapor deposition method is adopted for forming each layer, thevapor deposition conditions may be changed depending on the compoundsused. Generally, the following ranges are suitably selected for theconditions, heating temperature of boat: 50 to 450° C., level of vacuum:1×10⁻⁶ to 1×10⁻² Pa, vapor deposition rate: 0.01 to 50 nm/sec,temperature of substrate: −50 to 300° C., and layer thickness: 0.1 nm to5 μm, preferably 5 to 200 nm.

Formation of organic layers of the present invention is preferablycontinuously carried out from a hole injection layer to a cathode withone time vacuuming. It may be taken out on the way, and a differentlayer forming method may be employed. In that case, the operation ispreferably done under a dry inert gas atmosphere.

<<Anode>>

As an anode of an organic EL element, a metal having a large workfunction (4 eV or more, preferably, 4.5 eV or more), an alloy, and aconductive compound and a mixture thereof are utilized as an electrodesubstance.

Specific examples of an electrode substance are: metals such as Au, andan alloy thereof; transparent conductive materials such as CuI, indiumtin oxide (ITO), SnO₂, and ZnO. Further, a material such as IDIXO(In₂O₃—ZnO), which may form an amorphous and transparent electrode, mayalso be used.

As for an anode, these electrode substances may be made into a thinlayer by a method such as a vapor deposition method or a sputteringmethod; followed by making a pattern of a desired form by aphotolithography method. Otherwise, when the requirement of patternprecision is not so severe (about 100 μm or more), a pattern may beformed through a mask of a desired form at the time of layer formationwith a vapor deposition method or a sputtering method using theabove-described material.

Alternatively, when a coatable substance such as an organic conductivecompound is employed, it is possible to employ a wet film forming methodsuch as a printing method or a coating method. When emitted light istaken out from the anode, the transmittance is preferably set to be 10%or more. A sheet resistance of the anode is preferably a few hundredΩ/sq or less.

Further, although a layer thickness of the anode depends on a material,it is generally selected in the range of 10 nm to 1 μm, and preferablyin the range of 10 to 200 nm.

<<Cathode>>

As a cathode, a metal having a small work function (4 eV or less) (it iscalled as an electron injective metal), an alloy, a conductive compoundand a mixture thereof are utilized as an electrode substance. Specificexamples of the aforesaid electrode substance includes: sodium,sodium-potassium alloy, magnesium, lithium, a magnesium/copper mixture,a magnesium/silver mixture, a magnesium/aluminum mixture, amagnesium/indium mixture, an aluminum/aluminum oxide (Al₂O₃) mixture,indium, a lithium/aluminum mixture, aluminum, and a rare earth metal.Among them, with respect to an electron injection property anddurability against oxidation, preferable are: a mixture of electioninjecting metal with a second metal which is stable metal having a workfunction larger than the electron injecting metal. Examples thereof are:a magnesium/silver mixture, a magnesium/aluminum mixture, amagnesium/indium mixture, an aluminum/aluminum oxide (Al₂O₃) mixture, alithium/aluminum mixture and aluminum.

A cathode may be made by using these electrode substances with a methodsuch as a vapor deposition method or a sputtering method to form a thinfilm. A sheet resistance of the cathode is preferably a few hundred Q/sqor less. A layer thickness of the cathode is generally selected in therange of 10 nm to 5 μm, and preferably in the range of 50 to 200 nm.

In order to transmit emitted light, it is preferable that one of ananode and a cathode of an organic EL element is transparent ortranslucent for achieving an improved luminescence.

Further, after forming a layer of the aforesaid metal having a thicknessof 1 to 20 nm on the cathode, it is possible to prepare a transparent ortranslucent cathode by providing with a conductive transparent materialdescribed in the description for the anode thereon. By applying thisprocess, it is possible to produce an element in which both an anode anda cathode are transparent.

[Support Substrate]

A support substrate which may be used for an organic EL element of thepresent invention is not specifically limited with respect to types suchas glass and plastics. Hereafter, the support substrate may be alsocalled as substrate body, substrate, substrate substance, or support.They may be transparent or opaque. However, a transparent supportsubstrate is preferable when the emitting light is taken from the sideof the support substrate. Support substrates preferably utilizedincludes such as glass, quartz and transparent resin film. Aspecifically preferable support substrate is a resin film capable ofproviding an organic EL element with a flexible property.

Examples of a resin film include: polyesters such as polyethyleneterephthalate (PET) and polyethylene naphthalate (PEN), polyethylene,polypropylene, cellophane, cellulose esters and their derivatives suchas cellulose diacetate, cellulose triacetate (TAC), cellulose acetatebutyrate, cellulose acetate propionate (CAP), cellulose acetatephthalate, and cellulose nitrate, polyvinylidene chloride, polyvinylalcohol, polyethylene vinyl alcohol, syndiotactic polystyrene,polycarbonate, norbornene resin, polymethyl pentene, polyether ketone,polyimide, polyether sulfone (PES), polyphenylene sulfide, polysulfones,polyether imide, polyether ketone imide, polyamide, fluororesin, Nylon,polymethyl methacrylate, acrylic resin, polyallylates and cycloolefinresins such as ARTON (trade name, made by JSR Co. Ltd.) and APEL (tradename, made by Mitsui Chemicals, Inc.).

On the surface of a resin film, it may be formed a film incorporating aninorganic or an organic compound or a hybrid film incorporating bothcompounds. Barrier films are preferred with a water vapor permeabilityof 0.01 g/m²·24 h or less (at 25±0.5° C., and 90±2% RH) determined basedon JIS K 7129-1992. Further, high barrier films are preferred to have anoxygen permeability of 1×10⁻³ cm³/m²·24 h·atm or less determined basedon JIS K 7126-1987, and a water vapor permeability of 1×10⁻⁵ g/m²·24 hor less.

As materials that form a barrier film, employed may be those whichretard penetration of moisture and oxygen, which deteriorate theelement. For example, it is possible to employ silicon oxide, silicondioxide, and silicon nitride. Further, in order to improve thebrittleness of the aforesaid film, it is more preferable to achieve alaminated layer structure of inorganic layers and organic layers. Thelaminating order of the inorganic layer and the organic layer is notparticularly limited, but it is preferable that both are alternativelylaminated a plurality of times.

Barrier film forming methods are not particularly limited, and examplesof employable methods include a vacuum deposition method, a sputteringmethod, a reactive sputtering method, a molecular beam epitaxy method, acluster ion beam method, an ion plating method, a plasma polymerizationmethod, a plasma CVD method, a laser CVD method, a thermal CVD method,and a coating method. Of these, specifically preferred is a methodemploying an atmospheric pressure plasma polymerization method,described in JP-A No. 2004-68143.

Examples of opaque support substrates include metal plates such aluminumor stainless steel films, opaque resin substrates, and ceramicsubstrates.

An external taking out quantum efficiency of light emitted by theorganic EL element of the present invention is preferably at least 1% ata room temperature, but is more preferably at least 5%.

External taking out quantum efficiency (%)=(Number of photons emitted bythe organic EL element to the exterior/Number of electrons fed toorganic EL element)×100.

Further, it may be used simultaneously a color hue improving filter suchas a color filter, or it may be used simultaneously a color conversionfilter which convert emitted light color from the organic EL element tomulticolor by employing fluorescent materials.

[Sealing]

As sealing means employed in the present invention, listed may be, forexample, a method in which sealing members, electrodes, and a supportingsubstrate are subjected to adhesion via adhesives. The sealing membersmay be arranged to cover the display region of an organic EL element,and may be a concave plate or a flat plate. Neither transparency norelectrical insulation is limited.

Specifically listed are glass plates, polymer plate-films, metalplate-films. Specifically, it is possible to list, as glass plates,soda-lime glass, barium-strontium containing glass, lead glass,aluminosilicate glass, borosilicate glass, barium borosilicate glass,and quartz. Further, listed as polymer plates maybe polycarbonate,acryl, polyethylene terephthalate, polyether sulfide, and polysulfone.As a metal plate, listed are those composed of at least one metalselected from the group consisting of stainless steel, iron, copper,aluminum magnesium, nickel, zinc, chromium, titanium, molybdenum,silicon, germanium, and tantalum, or alloys thereof.

In the present invention, since it is possible to achieve a thin organicEL element, it is preferable to employ a polymer film or a metal film.Further, it is preferable that the polymer film has an oxygenpermeability of 1×10⁻³ cm³/m²·24 h·atm or less determined by the methodbased on JIS K 7126-1987, and a water vapor permeability of 1×10⁻³g/m²·24 h or less (at 25±0.5° C., and 90±2% RH) determined by the methodbased on JIS K 7129-1992.

Conversion of the sealing member into concave is carried out byemploying a sand blast process or a chemical etching process.

In practice, as adhesives, listed may be photo-curing and heat-curingtypes having a reactive vinyl group of acrylic acid based oligomers andmethacrylic acid, as well as moisture curing types such as2-cyanoacrylates. Further listed may be thermal and chemical curingtypes (mixtures of two liquids) such as epoxy based ones. Still furtherlisted may be hot-melt type polyamides, polyesters, and polyolefins. Yetfurther listed may be cationically curable type UV curable epoxy resinadhesives.

In addition, since an organic EL element is occasionally deterioratedvia a thermal process, preferred are those which enable adhesion andcuring between a room temperature and 80° C. Further, desiccating agentsmay be dispersed into the aforesaid adhesives. Adhesives may be appliedonto sealing portions via a commercial dispenser or printed on the samein the same manner as screen printing.

Further, it is appropriate that on the outside of the aforesaidelectrode which interposes the organic layer and faces the supportsubstrate, the aforesaid electrode and organic layer are covered, and inthe form of contact with the support substrate, inorganic and organicmaterial layers are formed as a sealing film. In this case, as materialsthat form the aforesaid film may be those which exhibit functions toretard penetration of moisture or oxygen which results in deterioration.For example, it is possible to employ silicon oxide, silicon dioxide,and silicon nitride.

Still further, in order to improve brittleness of the aforesaid film, itis preferable that a laminated layer structure is formed, which iscomposed of these inorganic layers and layers composed of organicmaterials. Methods to form these films are not particularly limited. Itis possible to employ, for example, a vacuum deposition method, asputtering method, a reactive sputtering method, a molecular beamepitaxy method, a cluster ion beam method, an ion plating method, aplasma polymerization method, an atmospheric pressure plasmapolymerization method, a plasma CVD method, a thermal CVD method, and acoating method.

It is preferable to inject a gas phase and a liquid phase material ofinert gases such as nitrogen or argon, and inactive liquids such asfluorinated hydrocarbon or silicone oil into the space between the spaceformed with the sealing member and the display region of the organic ELelement. Further, it is possible to form vacuum in the space. Stillfurther, it is possible to enclose hygroscopic compounds in the interiorof the space.

Examples of a hygroscopic compound include: metal oxides (for example,sodium oxide, potassium oxide, calcium oxide, barium oxide, magnesiumoxide, and aluminum oxide); sulfates (for example, sodium sulfate,calcium sulfate, magnesium sulfate, and cobalt sulfate); metal halides(for example, calcium chloride, magnesium chloride, cesium fluoride,tantalum fluoride, cerium bromide, magnesium bromide, barium iodide, andmagnesium iodide); perchlorates (for example, barium perchlorate andmagnesium perchlorate). In sulfates, metal halides, and perchlorates,suitably employed are anhydrides. For sulfate salts, metal halides andperchlorates, suitably used are anhydrous salts.

[Protective Film and Protective Plate]

On the aforesaid sealing film which interposes the organic layer andfaces the support substrate or on the outside of the aforesaid sealingfilm, a protective or a protective plate may be arranged to enhance themechanical strength of the element. Specifically, when sealing isachieved via the aforesaid sealing film, the resulting mechanicalstrength is not always high enough, therefore it is preferable toarrange the protective film or the protective plate described above.Usable materials for these include glass plates, polymer plate-films,and metal plate-films which are similar to those employed for theaforesaid sealing. However, from the viewpoint of reducing weight andthickness, it is preferable to employ a polymer film.

[Improving Method of Light Extraction]

It is generally known that an organic EL element emits light in theinterior of the layer exhibiting the refractive index (being about 1.6to 2.1) which is greater than that of air, whereby only about 15% to 20%of light generated in the light emitting layer is extracted. This is dueto the fact that light incident to an interface (being an interlace of atransparent substrate to air) at an angle of θ which is at leastcritical angle is not extracted to the exterior of the element due tothe resulting total reflection, or light is totally reflected betweenthe transparent electrode or the light emitting layer and thetransparent substrate, and light is guided via the transparent electrodeor the light emitting layer, whereby light escapes in the direction ofthe element side surface.

Means to enhance the efficiency of the aforesaid light extractioninclude, for example: a method in which roughness is formed on thesurface of a transparent substrate, whereby total reflection isminimized at the interface of the transparent substrate to air (U.S.Pat. No. 4,774,435), a method in which efficiency is enhanced in such amanner that a substrate results in light collection (JP-A No.63-314795), a method in which a reflection surface is formed on the sideof the element (JP-A No. 1-220394), a method in which a flat layer of amiddle refractive index is introduced between the substrate and thelight emitting body and an antireflection film is formed (JP-A No.62-172691), a method in which a flat layer of a refractive index whichis equal to or less than the substrate is introduced between thesubstrate and the light emitting body (JP-A No. 2001-202827), and amethod in which a diffraction grating is formed between the substrateand any of the layers such as the transparent electrode layer or thelight emitting layer (including between the substrate and the outside)(JP-A No. 11-283751).

In the present invention, it is possible to employ these methods whilecombined with the organic EL element of the present invention. Of these,it is possible to appropriately employ the method in which a flat layerof a refractive index which is equal to or less than the substrate isintroduced between the substrate and the light emitting body and themethod in which a diffraction grating is formed between any layers of asubstrate, and a transparent electrode layer and a light emitting layer(including between the substrate and the outside space).

By combining these means, the present invention enables the productionof elements which exhibit higher luminance or excel in durability.

When a low refractive index medium having a thickness, greater than thewavelength of light is formed between the transparent electrode and thetransparent substrate, the extraction efficiency of light emitted fromthe transparent electrode to the exterior increases as the refractiveindex of the medium decreases.

As materials of the low refractive index layer, listed are, for example,aerogel, porous silica, magnesium fluoride, and fluorine based polymers.Since the refractive index of the transparent substrate is commonlyabout 1.5 to 1.7, the refractive index of the low refractive index layeris preferably approximately 1.5 or less. More preferably, it is 1.35 orless.

Further, thickness of the low refractive index medium is preferably atleast two times of the wavelength in the medium. The reason is that,when the thickness of the low refractive index medium reaches nearly thewavelength of light so that electromagnetic waves escaped via evanescententer into the substrate, effects of the low refractive index layer arelowered.

The method in which the interface which results in total reflection or adiffraction grating is introduced in any of the media is characterizedin that light extraction efficiency is significantly enhanced. The abovemethod works as follows. By utilizing properties of the diffractiongrating capable of changing the light direction to the specificdirection different from diffraction via so-called Bragg diffractionsuch as primary diffraction or secondary diffraction of the diffractiongrating, of light emitted from the light entitling layer, light, whichis not emitted to the exterior due to total reflection between layers,is diffracted via introduction of a diffraction grating between anylayers or in a medium (in the transparent substrate and the transparentelectrode) so that light is extracted to the exterior.

It is preferable that the introduced diffraction grating exhibits atwo-dimensional periodic refractive index. The reason is as follows.Since light emitted in the light emitting layer is randomly generated toall directions, in a common one-dimensional diffraction gratingexhibiting a periodic refractive index distribution only in a certaindirection, light which travels to the specific direction is onlydiffracted, whereby light extraction efficiency is not sufficientlyenhanced.

However, by changing the refractive index distribution to atwo-dimensional one, light, which travels to all directions, isdiffracted, whereby the light extraction efficiency is enhanced.

A position to introduce a diffraction grating may be between any layersor in a medium (in a transparent substrate or a transparent electrode).However, a position near the organic light emitting layer, where lightis generated, is preferable. In this case, the cycle of the diffractiongrating is preferably from about ½ to 3 times of the wavelength of lightin the medium. The preferable arrangement of the diffraction grating issuch that the arrangement is two-dimensionally repeated in the form of asquare lattice, a triangular lattice, or a honeycomb lattice.

[Light Collection Sheet]

Via a process to arrange a structure such as a micro-lens array shape onthe light extraction side of the organic EL element of the presentinvention or via combination with a so-called light collection sheet,light is collected in the specific direction such as the front directionwith respect to the light emitting element surface, whereby it ispossible to enhance luminance in the specific direction.

In an example of the micro-lens array, square pyramids to realize a sidelength of 30 μm and an apex angle of 90 degrees are two-dimensionallyarranged on the light extraction side of the substrate. The side lengthis preferably 10 to 100 μm. When it is less than the lower limit,coloration occurs due to generation of diffraction effects, while whenit exceeds the upper limit, the thickness increases undesirably.

It is possible to employ, as a light collection sheet, for example, onewhich is put into practical use in the LED backlight of liquid crystaldisplay devices. It is possible to employ, as such a sheet, for example,the luminance enhancing film (BEF), produced by Sumitomo 3M Limited. Asshapes of a prism sheet employed may be, for example, Δ shaped stripesof an apex angle of 90 degrees and a pitch of 50 μm formed on a basematerial, a shape in which the apex angle is rounded, a shape in whichthe pitch is randomly changed, and other shapes.

Further, in order to control the light radiation angle from the lightemitting element, simultaneously employed may be a light diffusionplate-film. For example, it is possible to employ the diffusion film(LIGHT-UP), produced by Kimoto Co., Ltd.

[Applications]

It is possible to employ the organic EL element of the present inventionas display devices, displays, and various types of light emittingsources.

Examples of light emitting sources include: lighting apparatuses (homelighting and car lighting), clocks, backlights for liquid crystals, signadvertisements, signals, light sources of light memory media, lightsources of electrophotographic copiers, light sources of lightcommunication processors, and light sources of light sensors. Thepresent invention is not limited to them. It is especially effectivelyemployed as a backlight of a liquid crystal display device and alighting source.

If needed, the organic EL element of the present, invention may undergopatterning via a metal mask or an ink-jet printing method during filmformation. When the patterning is carried out, only an electrode mayundergo patterning, an electrode and a light emitting layer may undergopatterning, or all element layers may undergo patterning. Duringpreparation of the element, it is possible to employ conventionalmethods.

<Display Device>

A display device provided with an organic EL element of the presentinvention may emit a single color or multiple colors. Here, it will bedescribed a multiple color display device.

In case of a multiple color display device, a shadow mask is placedduring the formation of a light emitting layer, and a layer is formed asa whole with a vapor deposition method, a cast method, a spin coatingmethod, an inkjet method, and a printing method.

When patterning is done only to the light emitting layer, although thecoating method is not limited in particular, preferable methods are avapor deposition method, an inkjet method, a spin coating method, and aprinting method.

A constitution of an organic EL element provided for a display device isselected from the above-described examples of an organic EL elementaccording to the necessity.

The production method of an organic EL element is described as anembodiment of a production method of the above-described organic ELelement.

When a direct-current voltage is applied to the produced multiple colordisplay device, light emission may be observed by applying voltage of 2o 40 V by setting the anode to have a plus (+) polarity, and the cathodeto have a minus (−) polarity. When the voltage is applied to the devicewith reverse polarities, an electric current does not pass and lightemission does not occur. Further, when an alternating-current voltage isapplied to the device, light emission occurs only when the anode has aplus (+) polarity and the cathode has a minus (−) polarity. In addition,an arbitrary wave shape may be used for applying alternating-current.

The multiple color display device may be used for a display device, adisplay, and a variety of light emitting sources. In a display device ora display, a full color display is possible by using 3 kinds of organicEL elements emitting blue, red and green.

Examples of a display device or a display are: a television set, apersonal computer, a mobile device, an AV device, a character broadcastdisplay, and an information display in a car. Specifically, it may beused for a display device reproducing a still image or a moving image.When it is used for a display device reproducing a moving image, thedriving mode may be any one of a passive-matrix mode and anactive-matrix mode.

Examples of a light emitting source include: home lighting, carlighting, backlights for clocks and liquid crystals, signadvertisements, signals, light sources of light memory media, lightsources of electrophotographic copiers, light sources of lightcommunication processors, and light sources of light sensors. Thepresent invention is not limited to them.

In the following, an example of a display device provided with anorganic EL element of the present invention will be described byreferring to drawings.

FIG. 9 is a schematic drawing illustrating an example of a displaydevice composed of an organic EL element. Display of image informationis carried out by light emission of an organic EL element. For example,it is a schematic drawing of a display of a cell-phone.

A display 1 is constituted of a display section A having plural numberof pixels, a control section B which performs image scanning of thedisplay section A based on image information, and a wiring section Celectrically connecting the display section A and the control section B.

The control section B, which is electrically connected to the displaysection A via the wiring section C, sends a scanning signal and an imagedata signal to plural number of pixels based on image information fromthe outside and pixels of each scanning line successively emit dependingon the image data signal by a scanning signal to perform image scanning,whereby image information is displayed on the display section A.

FIG. 10 is a schematic drawing of the display section A based on anactive matrix mode.

The display section A is provided with the wiring section C, whichcontains plural scanning lines 5 and data lines 6, and plural pixels 3on a substrate. Primary part materials of the display section A will beexplained in the following.

In FIG. 10, illustrated is the case that light emitted by the pixel 3 istaken out along the white allow (downward).

The scanning lines 5 and the plural data lines 6 each are comprised of aconductive material, and the scanning lines 5 and the data lines 6 areperpendicular in a grid form and are connected to pixels 3 at theright-angled crossing points (details are not shown in the drawing).

The pixel 3 receives an image data from the data line 6 when a scanningsignal is applied from the scanning line 5 and emits according to thereceived image data.

Full-color display is possible by appropriately arranging pixels havingan emission color in a red region, pixels in a green region and pixelsin a blue region, side by side on the same substrate.

Next, an emission process of a pixel will be explained. FIG. 11 is aschematic drawing of a pixel.

A pixel is equipped with an organic EL element 10, a switchingtransistor 11, an operating transistor 12 and a capacitor 13. Red, greenand blue emitting organic EL elements are utilized as the organic ELelement 10 for plural pixels, and full-color display device is possibleby arranging these side by side on the same substrate.

In FIG. 11, an image data signal is applied on the drain of theswitching transistor 11 via the data line 6 from the control section B.Then when a scanning signal is applied on the gate of the switchingtransistor 11 via the scanning line 5 from control section B, operationof switching transistor is on to transmit the image data signal appliedon the drain to the gates of the capacitor 13 and the operatingtransistor 12.

The operating transistor 12 is on, simultaneously with the capacitor 13being charged depending on the potential of an image data signal, bytransmission of an image data signal. In the operating transistor 12,the drain is connected to an electric source line 7 and the source isconnected to the electrode of the organic EL element 10, and an electriccurrent is supplied from the electric source line 7 to the organic ELelement 10 depending on the potential of an image data applied on thegate.

When a scanning signal is transferred to the next scanning line 5 bysuccessive scanning of the control section B, operation of the switchingtransistor 11 is off.

However, since the capacitor 13 keeps the charged potential of an imagedata signal even when operation of the switching transistor 11 is off,operation of the operating transistor 12 is kept on to continue emissionof the organic EL element 10 until the next scanning signal is applied.

When the next scanning signal is applied by successive scanning, theoperating transistor 12 operates depending on the potential of an imagedata signal synchronized to the scanning signal and the organic ELelement 10 emits light.

That is, emission of each organic EL element 10 of the plural pixels 3is performed by providing the switching transistor 11 and the operatingtransistor 12 against each organic EL element 10 of plural pixels 3.Such an emission method is called as an active matrix mode.

Herein, emission of the organic EL element 10 may be either emission ofplural gradations based on a multiple-valued image data signal havingplural number of gradation potentials or on and off of a predeterminedemission quantity based on a binary image data signal. Further,potential hold of the capacitor 13 may be either continuously maintaineduntil the next scanning signal application or discharged immediatelybefore the next scanning signal application.

In the present invention, emission operation is not necessarily limitedto the above-described active matrix mode but may be a passive matrixmode in which organic EL element is emitted based on a data signal onlywhen a scanning signal is scanned.

FIG. 12 is a schematic drawing of a display device based on a passivematrix mode. In FIG. 12, plural number of scanning lines 5 and pluralnumber of image data lines 6 are arranged grid-wise, opposing to eachother and sandwiching the pixels 3.

When a scanning signal of the scanning line 5 is applied by successivescanning, the pixel 3 connected to the scanning line 5 applied with thesignal emits depending on an image data signal.

Since the pixel 3 is provided with no active element in a passive matrixmode, decrease of manufacturing cost is possible.

By employing the organic EL element of the present invention, it waspossible to obtain a display device having improved emission efficiency.

<Light Emitting Device>

An organic EL element of the present invention may be used for a lightemitting device.

An organic EL element of the present invention may be provided with arasonator structure. The intended uses of the organic EL elementprovided with a rasonator structure are: a light source of a lightmemory media, a light source of an electrophotographic copier, a lightsource of a light communication processor, and a light source of a lightsensor, however, it is not limited to them. It may be used for theabove-described purposes by making to emit a laser.

Further, an organic EL element of the present invention may be used fora kind of lamp such as for illumination or exposure. It may be used fora projection device for projecting an image, or may be used for adisplay device to directly observe a still image or a moving imagethereon.

The driving mode used for a display device of a moving imagereproduction may be any one of a passive matrix mode and an activematrix mode. By employing two or more kinds of organic EL elements ofthe present invention emitting a different emission color, it mayproduce a full color display device.

In addition, a π-conjugated compound used in the present invention maybe applicable to an organic EL element substantially emitting whitelight as a light emitting device. For example, when a plurality of lightemitting materials are employed, white light may be obtained by mixingcolors of a plurality of emission colors. As a combination of theplurality of emission colors, it may be a combination of red, green andblue having emission maximum wavelength of three primary colors, or itmay be a combination of colors having two emission maximum wavelengthmaking use of the relationship of two complementary colors of blue andyellow, or blue-green and orange.

A production method of an organic EL element of the present invention isdone by placing a mask only during formation of a light emitting layer,a hole transport layer and an electron transport layer. It may beproduced by coating with a mask to make simple arrangement. Since otherlayers are common, there is no need of pattering with a mask. Forexample, it may produce an electrode uniformly with a vapor depositionmethod, a cast method, a spin coating method, an inkjet method, and aprinting method. The production yield will be improved.

By using these methods, it may be produced a white organic EL device inwhich a plurality of light emitting elements are arranged in parallel toform an array state. The element itself emits white light.

One Embodiment of Lighting Device of the Present Invention

One embodiment of lighting devices of the present Invention providedwith an organic EL element of the present invention will be described.

The non-light emitting surface of the organic EL element of the presentinvention was covered with a glass case, and a 300 μm thick glasssubstrate was employed as a sealing substrate. An epoxy based lightcurable type adhesive (LUXTRACK LC0629B produced by Toagosei Co., Ltd.)was employed in the periphery as a sealing material. The resulting onewas superimposed on the aforesaid cathode to be brought into closecontact with the aforesaid transparent support substrate, and curing andsealing were carried out via exposure of UV radiation onto the glasssubstrate side, whereby the lighting device shown in FIG. 13 and FIG.14, was formed.

FIG. 13 is a schematic view of a lighting device, and an organic ELelement of the present invention (Organic EL element 101 in a lightemitting device) is covered with glass cover 102 (incidentally, sealingby the glass cover was carried out in a globe box under nitrogenambience (under an ambience of high purity nitrogen gas at a purity ofat least 99.999%) so that Organic EL Element 101 was not brought intocontact with atmosphere).

FIG. 14 is a cross-sectional view of a lighting device. In FIG. 6, 105represents a cathode, 106 represents an organic EL layer, and 107represents a glass substrate fitted with a transparent electrode.Further, the interior of glass cover 102 is filled with nitrogen gas 108and water catching agent 109 is provided.

By employing an organic EL element of the present invention, it waspossible to obtain a light emitting having improved emission efficiency.

<Light-Emitting Thin Film>

A light-emitting thin film of the present invention is characterized incontaining a π-conjugated compound according to the above-describedpresent invention. It may be produced in the same way as preparation ofthe above-described organic layer.

Forming methods of a light-emitting thin film according to the presentinvention are not specifically limited. They may be formed by using aknown method such as a vacuum vapor deposition method and a wet method(wet process).

Examples of a wet process include: a spin coating method, a cast method,an inkjet method, a printing method, a die coating method, a bladecoating method, a roll coating method, a spray coating method, a curtaincoating method, and a LB method (Langmuir Blodgett method). From theviewpoint of getting a uniform thin layer with high productivity,preferable are methods highly appropriate to a roll-to-roll method suchas a die coating method, a roll coating method, an inkjet method, and aspray coating method.

Examples of a liquid medium to dissolve or to disperse a π-conjugatedcompound according to the present invention include: ketones such asmethyl ethyl ketone and cyclohexanone; aliphatic esters such as ethylacetate; halogenated hydrocarbons such as dichlorobenzene; aromatichydrocarbons such as toluene, xylene, mesitylene, and cyclohexylbenzene;aliphatic hydrocarbons such as cyclohexane, decalin, and dodecane;organic solvents such as DMF and DMSO.

These will be dispersed with a dispersion method such as an ultrasonicdispersion method, a high shearing dispersion method and a mediadispersion method.

A different film forming method may be applied to every organic layer.When a vapor deposition method is adopted for forming each layer, thevapor deposition conditions will change depending on the compounds used.Generally, the following ranges are suitably selected for theconditions, heating temperature of boat: 50 to 450° C., level of vacuum:1×10⁻⁶ to 1×10⁻² Pa, vapor deposition rate: 0.01 to 50 nm/sec,temperature of substrate: −50 to 300° C., and layer thickness: 0.1 nm to5 μm, preferably 5 to 200 nm.

When a spin coating method is adopted, it is preferable to use a spincoater in the range of 100 to 1000 rpm for 10 to 120 seconds under a dryinert gas atmosphere.

A light-emitting thin film of the present invention may be used for adisplay device or a light emitting device.

EXAMPLES

Hereafter, the present invention will be described specifically byreferring to examples, however, the present invention is not limited tothem. In examples, the indication of “%” is used. Unless particularlymentioned, it represents “mass %”.

A calculation method of an angle θ in Examples will be described byreferring to a compound T-93 as an example. The compound T-93 wascalculated by performing an optimization of a structure with adensity-functional calculation method using B3LYP as a functional and6-31G(d) as a base function. The theoretical calculation resultsrevealed that a LUMO is localized at a dicyanobenzene being an acceptorportion, and a HOMO is localized at a 9,10-dihydroacrydine portion beinga donor portion. Since the frontier orbitals of the compound T-93 are aπ*-orbital and a π-orbital, as described above, a vertical direction tothe π-conjugated plane is a direction vector of the LUMO orbital and theHOMO orbital defined in the present invention (refer to FIG. 15). Byusing the optimized structure obtained by the theoretical calculation,the angle θ formed with two direction vectors was calculated to be 164degree for the compound T-93.

Example 1

(Preparation of Organic EL Element 1-1)

An anode was prepared on a glass substrate of 50 mm×50 mm with athickness of 0.7 mm by forming a film of ITO (indium tin oxide) with athickness of 150 nm, then by making patterning to it. The transparentsupport substrate provided with the ITO transparent electrode wassubjected to ultrasonic washing with isopropyl alcohol, followed bydrying with desiccated nitrogen gas, and it was subjected to UV ozonewashing for 5 minutes. The transparent support substrate was fixed to asubstrate holder of a commercial vacuum deposition apparatus.

The constituting materials for each layer were loaded in each heatingboat for vapor deposition in the vacuum deposition apparatus with anoptimum amount. As a heating boat for vapor deposition, it was used aresistance heating boat made of molybdenum or tungsten.

After reducing the pressure of a vacuum tank to 4×10⁻⁴ Pa, the heatingboat containing HAT-CN (1,4,5,8,9,12-hexaazatriphenylenehexacarbonitrile) was heated via application of electric current andvapor deposition was made onto the ITO transparent electrode at adeposition rate of 0.1 nm/second, whereby it was produced a holeinjection layer having a thickness of 10 nm.

Subsequently, α-NPD (4,4′-bis[N-(naphthyl)-N-phenylamino]biphenyl) wasdeposited onto the hole injection layer at a deposition rate of 0.1nm/second, whereby it was produced a hole transport layer having athickness of 40 nm.

Further, a host compound mCP (1,3-bis(N-carbazolyl) benzene) and acomparative compound 1 were co-deposited onto the hole transport layerat a deposition rate of 0.1 nm/second so that they have 96 volume % and4 volume % respectively, whereby it was produced a light emitting layerhaving a thickness of 30 nm.

Subsequently, BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) wasdeposited at a deposition rate of 0.1 nm/second, whereby it was producedan electron transport layer having a thickness of 30 nm.

Further, after forming a lithium fluoride layer having a thickness of0.5 nm, 110 nm thick aluminum was vapor deposited to form a cathode.

The non-light emitting surface side of the produced element was sealedby a glass case having a can shape under an ambience of high puritynitrogen gas having a purity of at least 99.999%. The electrode takenout wiring was set to obtain an organic EL element 1-1.

(Preparation of Organic EL Elements 1-2 to 1-9)

Organic EL elements 1-2 to 1-9 were prepared in the same manner aspreparation of the organic EL element 1-1 except that the light emittingcompound was changed from the comparative compound 1 to the compoundsdescribed in Table 1.

TABLE 1 Emission Organic EL Emission Angle θ ΔE_(ST) efficiency elementNo. compound (degree) (eV) (%) Remarks 1-1 Comparative 50 0.47 100 Comp.compound 1 1-2 T-71 130 0.18 109 Inv. 1-3 T-19 135 0.02 114 Inv. 1-4 T-2120 0.04 111 Inv. 1-5 T-86 141 0.03 117 Inv. 1-6 T-92 110 0.03 111 Inv.1-7 T-93 164 0.01 121 Inv. 1-8 T-96 113 0.04 110 Inv. 1-9 Comparative 710.07 103 Comp. compound 2 Comp.: Comparative example Inv.: Inventiveexample

Example 2

(Preparation of Organic EL Element 2-1)

An anode was prepared by making patterning to a glass substrate of 100mm×100 mm×1.1 mm (NA45, produced by NH Techno Glass Corp.) on which ITO(indium tin oxide) was formed with a thickness of 100 nm. Thereafter,the above transparent support substrate provided with the ITOtransparent electrode was subjected to ultrasonic washing with isopropylalcohol, followed by drying with desiccated nitrogen gas, and it wassubjected to UV ozone washing for 5 minutes.

On the transparent support substrate thus prepared was applied a 70%solution of poly (3,4-ethylenedioxythiphene)-polystyrene sulfonate(PEDOT/PSS, Baytron P AI4083, made by Bayer AG.) diluted with water byusing a spin coating method at 3,000 rpm for 30 seconds to form a film,and then it was dried at 200° C. for one hour. A hole injection layerhaving a thickness of 20 nm was prepared. The resulting transparentsupport substrate was fixed to a substrate holder of a commercial vacuumdeposition apparatus. The constituting materials for each layer wereloaded in each heating boat for vapor deposition in the vacuumdeposition apparatus with an optimum amount. As a heating boat for vapordeposition, it was used a resistance heating boat made of molybdenum ortungsten.

After reducing the pressure of a vacuum tank to 4×10⁻⁴ Pa, α-NPD wasdeposited onto the hole injection layer at a deposition rate of 0.1nm/second, whereby it was produced a hole transport layer having athickness of 40 nm.

CDBP and perylene were co-deposited onto the hole transport layer at adeposition rate of 0.1 nm/second so that they have 94 volume % and 6volume % respectively, whereby it was produced a light emitting layerhaving a thickness of 30 nm.

Subsequently, TPBi (1,3,5-tris (N-benzimidazole-2-yl)) was deposited ata deposition rate of 0.1 nm/second, whereby it was produced an electrontransport layer having a thickness of 30 nm.

Further, after forming a lithium fluoride layer having a thickness of0.5 nm, 110 nm thick aluminum was vapor deposited to form a cathode.

The non-light emitting surface side of the produced element was sealedby a glass case having a can shape under an ambience of high puritynitrogen gas having a purity of at least 99.999%. The electrode takenout wiring was set to obtain an organic EL element 2-1.

(Preparation of Organic EL Element 2-2)

Organic EL elements 2-2 was prepared in the same manner as preparationof the organic EL element 2-1 except that the light emitting layer wasformed by using: CDBP as a host compound; perylene as a light emittingcompound; and the comparative compound 1 as a third compound, and thecontents of compounds were respectively adjusted to be 80 volume %, 6volume %, and 14 volume %.

(Preparation of Organic EL Elements 2-3 to 2-9)

Organic EL elements 2-3 to 2-9 were prepared in the same manner aspreparation of the organic EL element 2-2 except that the third compoundwas changed as indicated in Table 2.

TABLE 2 Emission Organic EL Third Angle θ ΔE_(ST) efficiency element No.component (degree) (eV) (%) Remarks 2-1 None — — 100 Comp. 2-2Comparative 50 0.47 105 Comp. compound 1 2-3 T-71 130 0.18 115 Inv. 2-4T-19 135 0.02 119 Inv. 2-5 T-2 120 0.04 118 Inv. 2-6 T-86 141 0.03 125Inv. 2-7 T-93 164 0.01 130 Inv. 2-8 Comparative 71 0.07 108 Comp.compound 2 2-9 T-110 120 0.01 112 Inv. Comp.: Comparative example Inv.:Inventive example

Example 3

(Preparation of Organic EL Element 3-1)

An anode was prepared on a glass substrate of 50 mm×50 mm with athickness of 0.7 mm by forming a film of ITO (indium tin oxide) with athickness of 150 nm, then by making patterning to it. The transparentsupport substrate provided with the ITO transparent electrode wassubjected to ultrasonic washing with isopropyl alcohol, followed bydrying with desiccated nitrogen gas, and it was subjected to UV ozonewashing for 5 minutes. The transparent support substrate was fixed to asubstrate holder of a commercial vacuum deposition apparatus.

The constituting materials for each layer were loaded in each heatingboat for vapor deposition in the vacuum deposition apparatus with anoptimum amount. As a heating boat for vapor deposition, it was used aresistance heating boat made of molybdenum or tungsten.

After reducing the pressure of a vacuum tank to 4×10⁻⁴ Pa, the heatingboat containing HAT-CN (1,4,5,8,9,12-hexaazatriphenylenehexacarbonitrile) was heated via application of electric current anddeposition was made onto the ITO transparent electrode at a depositionrate of 0.1 nm/second, whereby it was produced a hole injection layerhaving a thickness of 15 nm.

Subsequently, α-NPD (4,4′-bis[N-(naphthyl)-N-phenylamino]biphenyl) wasdeposited onto the hole injection layer at a deposition rate of 0.1nm/second, whereby it was produced a hole transport layer having athickness of 30 nm.

Subsequently, the heating boats each containing the comparative compound1 as a host compound and tris (2-phenylpyridinate) iridium (III) wereheated via application of electric current and co-deposition was madeonto the hole transport layer at a deposition rate of 0.1 nm/second and0.010 nm/second, whereby it was produced a light emitting layer having athickness of 40 nm.

Subsequently, HB-1 was deposited at a deposition rate of 0.1 nm/second,whereby it was produced a first electron transport layer having athickness of 5 nm.

Further, ET-1 was deposited thereon at a deposition rate of 0.1nm/second, whereby it was produced a second electron transport layerhaving a thickness of 45 nm.

Further, after forming a lithium fluoride layer having a thickness of0.5 nm, 100 nm thick aluminum was vapor deposited to form a cathode.Thus, an organic EL element 3-1 was prepared.

(Preparation of Organic EL Elements 3-2 to 3-7)

Organic EL elements 3-2 to 3-7 were prepared in the same manner aspreparation of the organic EL element 3-1 except that the host compoundwas changed as indicated in Table 3.

In the same manner as described above, an emission luminance of theorganic EL element 3-1 was measured. A relative emission luminance ofeach organic EL element was obtained with respect to the emissionluminance of the organic EL element 3-1. The obtained measurementresults are listed in Table 3.

TABLE 3 Emission Organic EL Host Angle θ ΔE_(ST) efficiency element No.component (degree) (eV) (%) Remarks 3-1 Comparative 50 0.47 100 Comp.compound 1 3-2 Comparative 71 0.07 87 Comp. compound 2 3-3 T-71 130 0.18115 Inv. 3-4 T-86 141 0.03 118 Inv. 3-5 T-93 164 0.01 123 Inv. 3-6 T-97142 0.18 117 Inv. 3-7 T-101 110 0.01 109 Inv. Comp.: Comparative exampleInv.: Inventive example(Measurement of Emission Efficiency)

Emission efficiency of an organic EL element sample during driving wasevaluated by conducting the following measurement.

(Measurement of Emission Efficiency)

Each organic EL element thus produced was allowed to emit light byapplying a constant electric current of 2.5 mA/cm² at room temperature(about 25° C.). The emission luminance immediately after starting toemit light was measured with Spectroradiometer CS-2000 (produced byKonica Minolta, Inc.). The emission efficiency was determined. Theobtained results were indicated as a relative value in Tables 1 to 3.

In Example 1, the indicated values were a relative value when theemission efficiency of the organic EL element 1-1 was set to be 100%. InExample 2, the indicated values were a relative value when the emissionefficiency of the organic EL element 2-1 was set to be 100%. Further, inExample 3, the indicated values were a relative value when the emissionefficiency of the organic EL element 3-1 was set to be 100%.

CONCLUSION

An absolute value of the difference between the first (lowest) singletexcited level and the first (lowest) triplet excited level (ΔE_(ST)) wascalculated based on an optimized structure for calculation of an angleθ. The calculation of the excited state was done using Time-dependentdensity-functional calculation method (DFT) with Gaussian 09 using B3LYPas a functional and 6-31G(d) as a base function.

In Table 1, the organic EL elements 1-2 to 1-8 had a larger θ value anda smaller ΔE_(ST) value compared with the organic EL elements 1-1 and1-9. It was shown that the organic EL elements 1-2 to 1-8 exhibitedimproved emission efficiency.

In Table 2, it was shown that the organic EL elements 2-2 and 2-8containing a third component exhibited improved emission efficiencycompared with the organic EL elements 2-1. It was shown that the organicEL elements 2-3 to 2-7 and 2-9 containing a third component having alarger angle θ exhibited further improved emission efficiency comparedwith the organic EL elements 2-1.

In Table 3, it was shown that the organic EL elements 3-3 and 3-7 had alarger θ value and a smaller ΔE_(ST) value compared with the organic ELelements 3-1 and 3-2. The organic EL elements 3-3 and 3-7 exhibitedimproved emission efficiency.

INDUSTRIAL APPLICABILITY

As described above, the present invention is suitable to provide anorganic electroluminescent element enabling to achieve restrainedbroadening of an absorption spectrum and an emission spectrum, and highemission efficiency without using a rare metal.

DESCRIPTION OF SYMBOLS

-   1: Display-   3: Pixel-   5: Scanning line-   6: Data line-   7: Electric source line-   10: Organic EL element-   11: Switching transistor-   12: Operating transistor-   13: Capacitor-   101: Organic EL element in a light emitting device-   102: Glass cover-   105: Cathode-   106: Organic EL layer-   107: Glass substrate having a transparent electrode-   108: Nitrogen gas-   109: Water catching agent-   A: Display section-   B: Control section-   C: Wiring section

The invention claimed is:
 1. An organic electroluminescent elementcomprising an organic layer interposed between an anode and a cathode,the organic layer containing at least one light emitting layer, whereinthe at least one light emitting layer contains a π-conjugated compoundhaving an electron donor portion and an electron acceptor portion in themolecule; the π-conjugated compound has a direction vector from an atomhaving a HOMO orbital in the electron donor portion to an electron cloudof the HOMO orbital, and a direction vector from an atom having a LUMOorbital in the electron acceptor portion to an electron cloud of theLUMO orbital, and the two direction vectors form an angle θ in the rangeof 90 to 180 degrees; and the π-conjugated compound has a plurality ofthe electron donor portions or a plurality of the electron acceptorportions, and the π-conjugated compound is represented by any one ofFormulas (1) and (4):

wherein, X¹, X⁴, Y¹ to Y⁴, and Y¹⁰ to Y¹³ each respectively representthe electron donor portion or the electron acceptor portion; when X¹ andX⁴ each respectively represent the electron donor portion, Y¹ to Y⁴ andY¹⁰ to Y¹³ each respectively represent the electron acceptor portion;when X¹ and X⁴ each respectively represent the electron acceptorportion, Y¹ to Y⁴ and Y¹⁰ to Y¹³ each respectively represent theelectron donor portion; L¹ to L⁴ and represent a linking group, L¹ to L⁴each respectively represent an aryl group which may have a substituentor a heteroaryl group which may have a substituent, L¹ binds X¹ and Y¹through adjacent carbon atoms, L² binds X¹ and Y² through adjacentcarbon atoms, L³ binds X¹ and Y³ through adjacent carbon atoms, and L⁴binds X¹ and Y⁴ through adjacent carbon atoms.
 2. The organicelectroluminescent element described in claim 1, wherein the angle θ isin the range of 135 to 180 degrees.
 3. The organic electroluminescentelement described in claim 1, wherein one of the electron acceptorportions is bonded to two or more electron donor portions through thelinking group, or one of the electron donor portions is bonded to two ormore electron acceptor portions through the linking group.
 4. Theorganic electroluminescent element described in claim 1, wherein one ofthe electron acceptor portions is directly bonded to two or moreelectron donor portions, or one of the electron donor portions isdirectly bonded to two or more electron acceptor portions.
 5. Theorganic electroluminescent element described in claim 1, wherein theelectron donor portion and the electron acceptor portion represented byX¹, X⁴, Y¹ to Y⁴, and Y¹⁰ to Y¹³ in Formulas (1) and (4) eachrespectively are one selected from the group consisting of an aryl groupwhich may have a substituent, a heteroaryl group which may have asubstituent, an alkyl group which may have a substituent, a carbonylgroup which may have a substituent, a nitrogen atom which may have asubstituent, a sulfur atom which may have a substituent, a boron atomwhich may have a substituent, a phosphor atom which may have asubstituent, an oxygen atom which may have a substituent, and a siliconatom which may have a substituent.
 6. The organic electroluminescentelement described in claim 1, wherein L¹ to L⁴ in Formula (1) each are abenzene ring.
 7. The organic electroluminescent element described inclaim 1, wherein an absolute value of a difference between a lowestexcited singlet energy level and a lowest triplet energy level (ΔE_(ST))is 0.5 eV or less.
 8. The organic electroluminescent element describedin claim 1, wherein the at least one light emitting layer contains: theπ-conjugated compound; and at least one of a fluorescent compound and aphosphorescent compound.
 9. The organic electroluminescent elementdescribed in claim 1, wherein the at least one light emitting layercontains: the π-conjugated compound; at least one of a fluorescentcompound and a phosphorescent compound; and a host compound.
 10. Adisplay device provided with the organic electroluminescent elementdescribed in claim
 1. 11. A lighting device provided with the organicelectroluminescent element described in claim
 1. 12. A π-conjugatedcompound having an electron donor portion and an electron acceptorportion in the molecule, wherein the π-conjugated compound has adirection vector from an atom having a HOMO orbital in the electrondonor portion to an electron cloud of the HOMO orbital, and a directionvector from an atom having a LUMO orbital in the electron acceptorportion to an electron cloud of the LUMO orbital, and the two directionvectors form an angle θ in the range of 90 to 180 degrees; and theπ-conjugated compound has a plurality of the electron donor portions ora plurality of the electron acceptor portions, and the π-conjugatedcompound is represented by any one of Formulas (1) and (4):

wherein, X¹, X⁴, Y¹ to Y⁴, and Y¹⁰ to Y¹³ each respectively representthe electron donor portion or the electron acceptor portion; when X¹ andX⁴ each respectively represent the electron donor portion, Y¹ to Y⁴ andY¹⁰ to Y¹³ each respectively represent the electron acceptor portion;when X¹ and X⁴ each respectively represent the electron acceptorportion, Y¹ to Y⁴ and Y¹⁰ to Y¹³ each respectively represent theelectron donor portion; L¹ to L⁴ represent a linking group, L¹ to L⁴each respectively represent an aryl group which may have a substituentor a heteroaryl group which may have a substituent, L¹ binds X¹ and Y¹through adjacent carbon atoms, L² binds X¹ and Y² through adjacentcarbon atoms, L³ binds X¹ and Y³ through adjacent carbon atoms, and L⁴binds X¹ and Y⁴ through adjacent carbon atoms.
 13. A light-emitting thinfilm containing the π-conjugated compound described in claim 12.